Colour Basics

Electromagnetic Spectrum Visible Light	400 nanometres                        550 nanometres      700 nanometers
	The visible portion of the electromagnetic spectrum extends from about 400nm (violet light) to about 700nm (red light) with our eyes having peak sensitivity at about 550nm (green light).
	Colour In Image Files
	The remainder of this document has been sourced from the Adobe Photoshop CS3 Help Files.
Understanding Colour	Knowing how colors are created and how they relate to each other lets you work more effectively in Photoshop. Instead of achieving an effect by accident, you’ll produce consistent results thanks to an understanding of basic color theory. Primary colors Additive primaries are the three colors of light (red, green, and blue) that produce all the colors in the visible spectrum when added together in different combinations. Adding equal parts of red, blue, and green light produces white. The complete absence of red, blue, and green light results in black. Computer monitors are devices that use the additive primaries to create color.
	Additive colors  (RGB) R. Red G. Green B. Blue
	Subtractive primaries are pigments, which create a spectrum of colors in different combinations. Unlike monitors, printers use subtractive primaries (cyan, magenta, yellow, and black pigments) to produce colors through subtractive mixing. The term “subtractive” is used because the primary colors are pure until you begin mixing them together, resulting in colors that are less pure versions of the primaries. For example, orange is created through the subtractive mixing of magenta and yellow together.
	Subtractive colors (CMYK) C. Cyan M. Magenta Y. Yellow K. Black
	The color wheel If you’re new to adjusting color components, it helps to keep a standard color wheel diagram on hand when you work on color balance. You can use the color wheel to predict how a change in one color component affects other colors and also how changes translate between RGB and CMYK color models.				Color wheel R. Red Y. Yellow G. Green C. Cyan B. Blue M. Magenta
	For example, you can decrease the amount of any color in an image by increasing the amount of its opposite on the color wheel—and vice versa. Colors that lie opposite each other on the standard color wheel are known as complementary colors. Similarly, you can increase and decrease a color by adjusting the two adjacent colors on the wheel, or even by adjusting the two colors adjacent to its opposite. In a CMYK image, you can decrease magenta either by decreasing the amount of magenta or by increasing its complement, which is green (the color on the opposite side of the color wheel from magenta). In an RGB image, you can decrease magenta by removing red and blue or by adding green. All of these adjustments result in an overall color balance containing less magenta.

Colour Models, Spaces and Modes	A color model describes the colors we see and work with in digital images. Each color model, such as RGB, CMYK, or HSB, represents a different method (usually numeric) for describing color. A color space is a variant of a color model and has a specific gamut (range) of colors. For example, within the RGB color model are a number of color spaces: Adobe RGB, sRGB, ProPhoto RGB, and so on. Each device, like your monitor or printer, has its own color space and can only reproduce colors in its gamut. When an image moves from one device to another, image colors may change because each device interprets the RGB or CMYK values according to its own color space. You can use color management when moving images to ensure that most colors are the same or similar enough so they appear consistent. In Photoshop, a document’s color mode determines which color method is used to display and print the image you’re working on. Photoshop bases its color modes on the color models that are useful for images used in publishing. You can choose from RGB (Red, Green, Blue), CMYK (Cyan, Magenta, Yellow, Black), Lab Color (based on CIE L* a* b*), and Grayscale. Photoshop also includes modes for specialized color output such as Indexed Color and Duotone. Color modes determine the number of colors, the number of channels, and the file size of an image. Choosing a color mode also determines which tools and file formats are available. When you work with the colors in an image, you are adjusting numerical values in the file. It’s easy to think of a number as a color, but these numerical values are not absolute colors in themselves—they only have a color meaning within the color space of the device that is producing the color.
Adjusting Color Hue, Saturation, and Brightness	Based on the human perception of color, the HSB model describes three fundamental characteristics of color: Hue Color reflected from or transmitted through an object. It is measured as a location on the standard color wheel, expressed as a degree between 0° and 360°. In common use, hue is identified by the name of the color, such as red, orange, or green. Saturation Strength or purity of the color (sometimes called chroma). Saturation represents the amount of gray in proportion to the hue, measured as a percentage from 0% (gray) to 100% (fully saturated). On the standard color wheel, saturation increases from the center to the edge. Brightness Relative lightness or darkness of the color, usually measured as a percentage from 0% (black) to 100% (white).	HSB color model H. Hue S. Saturation B. Brightness

Colour Modes	Following are some of the major colour modes used :-
	Bitmap Mode Grayscale mode RGB Color Mode CMYK Color Mode	Indexed Color Mode Lab Color Mode Duotone Mode Multichannel Mode
Bitmap Mode	Bitmap mode uses one of two color values (black or white) to represent the pixels in an image. Images in Bitmap mode are called bitmapped 1‑bit images because they have a bit depth of 1.
Greyscale Mode	Grayscale mode uses different shades of gray in an image. In 8‑bit images, there can be up to 256 shades of gray. Every pixel of a grayscale image has a brightness value ranging from 0 (black) to 255 (white). In 16 and 32‑bit images, the number of shades in an image is much greater than in 8‑bit images. Grayscale values can also be measured as percentages of black ink coverage (0% is equal to white, 100% to black). Grayscale mode uses the range defined by the working space setting that you specify in the Color Settings dialog box.
RGB Colour Mode	Photoshop RGB Color mode uses the RGB model, assigning an intensity value to each pixel. In 8‑bits-per-channel images, the intensity values range from 0 (black) to 255 (white) for each of the RGB (red, green, blue) components in a color image. For example, a bright red color might have an R value of 246, a G value of 20, and a B value of 50. When the values of all three components are equal, the result is a shade of neutral gray. When the values of all components are 255, the result is pure white; when the values are 0, pure black. RGB images use three colors, or channels, to reproduce colors on-screen. In 8‑bits-per-channel images, the three channels translate to 24 (8 bits x 3 channels) bits of color information per pixel. With 24‑bit images, the three channels can reproduce up to 16.7 million colors per pixel. With 48‑bit (16‑bits-per-channel) and 96‑bit (32‑bits-per-channel) images, even more colors can be reproduced per pixel. In addition to being the default mode for new Photoshop images, the RGB model is used by computer monitors to display colors. This means that when working in color modes other than RGB, such as CMYK, Photoshop interpolates the CMYK image to RGB for display on-screen. Although RGB is a standard color model, the exact range of colors represented can vary, depending on the application or display device. The RGB Color mode in Photoshop varies according to the working space setting that you specify in the Color Settings dialog box.
CMYK Colour Mode	In the CMYK mode, each pixel is assigned a percentage value for each of the process inks. The lightest (highlight) colors are assigned small percentages of process ink colors; the darker (shadow) colors higher percentages. For example, a bright red might contain 2% cyan, 93% magenta, 90% yellow, and 0% black. In CMYK images, pure white is generated when all four components have values of 0%. Use the CMYK mode when preparing an image to be printed using process colors. Converting an RGB image into CMYK creates a color separation. If you start with an RGB image, it’s best to edit first in RGB and then convert to CMYK at the end of your process. In RGB mode, you can use the Proof Setup commands to simulate the effects of a CMYK conversion without changing the actual image data. You can also use CMYK mode to work directly with CMYK images scanned or imported from high-end systems. Although CMYK is a standard color model, the exact range of colors represented can vary, depending on the press and printing conditions. The CMYK Color mode in Photoshop varies according to the working space setting that you specify in the Color Settings dialog box.
Indexed Colour Mode	Indexed Color mode produces 8‑bit image files with up to 256 colors. When converting to indexed color, Photoshop builds a color lookup table (CLUT), which stores and indexes the colors in the image. If a color in the original image does not appear in the table, the program chooses the closest one or uses dithering to simulate the color using available colors. Although its palette of colors is limited, indexed color can reduce file size yet maintain the visual quality needed for multimedia presentations, web pages, and the like. Limited editing is available in this mode. For extensive editing, you should convert temporarily to RGB mode. Indexed color files can be saved in Photoshop, BMP, DICOM, GIF, Photoshop EPS, Large Document Format (PSB), PCX, Photoshop PDF, Photoshop Raw, Photoshop 2.0, PICT, PNG, Targa, or TIFF formats.
Lab Colour Mode	The CIE L*a*b* color model (Lab) is based on the human perception of color. The numeric values in Lab describe all the colors that a person with normal vision sees. Because Lab describes how a color looks rather than how much of a particular colorant is needed for a device (such as a monitor, desktop printer, or digital camera) to produce colors, Lab is considered to be a device-independent color model. Color management systems use Lab as a color reference to predictably transform a color from one color space to another color space. The Lab Color mode has a lightness component (L) that can range from 0 to 100. In the Adobe Color Picker and Color palette, the a component (green-red axis) and the b component (blue-yellow axis) can range from +127 to –128. Lab images can be saved in Photoshop, Photoshop EPS, Large Document Format (PSB), Photoshop PDF, Photoshop Raw, TIFF, Photoshop DCS 1.0, or Photoshop DCS 2.0 formats. You can save 48‑bit (16‑bits-per-channel) Lab images in Photoshop, Large Document Format (PSB), Photoshop PDF, Photoshop Raw, or TIFF formats. Note: The DCS 1.0 and DCS 2.0 formats convert the file to CMYK when opened.
Duotone Mode	Duotone mode creates monotone, duotone (two-color), tritone (three-color), and quadtone (four-color) grayscale images using one to four custom inks.
Multichannel Mode	Multichannel mode images contain 256 levels of gray in each channel and are useful for specialized printing. Multichannel mode images can be saved in Photoshop, Large Document Format (PSB), Photoshop 2.0, Photoshop Raw, or Photoshop DCS 2.0 formats. These guidelines apply when converting images to Multichannel mode:         Color channels in the original image become spot color channels in the converted image.         Converting a CMYK image to Multichannel mode creates cyan, magenta, yellow, and black spot channels.         Converting an RGB image to Multichannel mode creates cyan, magenta, and yellow spot channels.         Deleting a channel from an RGB, CMYK, or Lab image automatically converts the image to Multichannel mode.         To export a multichannel image, save it in Photoshop DCS 2.0 format.

Bit Depth

Bit depth specifies how much color information is available for each pixel in an image. The more bits of information per pixel, the more available colors and more accurate color representation. For example, an image with a bit depth of 1 has pixels with two possible values: black and white. An image with a bit depth of 8 has 28, or 256, possible values. Grayscale mode images with a bit depth of 8 have 256 possible gray values. RGB images are made of three color channels. An 8 bit per pixel RGB image has 256 possible values for each channel, which means it has over 16 million possible color values. RGB images with 8 bits per channel (bpc) are sometimes called 24 bit images (8 bits x 3 channels = 24 bits of data for each pixel). In addition to 8 bpc images, Photoshop can also work with images that contain 16 bpc or 32 bpc. Images with 32 bpc are also known as high dynamic range (HDR) images.