RGB and CMYK
In digital programs, two different sets of primary colors come into play: RGB
(red, green, and blue) and CMYK (cyan, magenta, yellow, and black). These
two distinct systems represent the two modes of color application: light and pigment.
Red, green, and blue are the primary colors familiar to lighting designers. The color theory that underlies the manipulation of light is called additive. Because screen images appear in direct, rather than reflective light, the colors we see there are made additively by combining red, blue, and green light. If you are only familiar with mixing pigments, you may find creating color in RGB mode somewhat counterintuitive. In additive color mixing, red plus green makes yellow (fig. 10.4). Moreover, when colored lights are combined, the color at the overlap is more luminous than the two colors that are its parents. The combination of all three primaries at full strength produces a white light.
The CMYK mode on the computer monitor is designed to simulate subtractive color mixing and represents the primary triad we know from mixing pigment. Figure 10.5 shows how the subtractive primaries form a secondary triad when intermixed. The combination of all three colors at the center is a dark chromatic gray, not black. Black, or “K tone,” is added to make a true black possible and to expand the range of tonal possibilities. Digital output, i.e. a printed version of a digital document, is made with the subtractive primaries. Printing processes use translucent inks instead of paint. In four-color and inkjet printing, cyan, magenta, and yellow (plus black) are overlapped and juxtaposed in tiny fragments for the eye to mix optically into what appear to be continuous tones.
The range of colors visually available in any system is called a GAMUT. The largest gamut is that of the colors we observe in the real world. Colors on a computer monitor are, in all modes, more limited in number than those in the visible spectrum.
The diagram at bottom right (fig. 10.6) shows the range of three color gamuts. The largest area, underlying the two linear configurations, is the visible spectrum. The second largest, represented by a pink triangle, is the RGB gamut. The smallest area, bound by a yellow line, is the CMYK gamut. Because the RGB gamut exceeds that of CMYK, there are colors that can appear on the computer monitor that will not be reproducible in printed form. (Adobe programs give a warning sign – ! – when a color created in RGB mode falls outside the CMYK gamut.) Bear in mind that colors appear more vivid on a monitor than they do in print. Also, subtle color relationships are more perceptible in print than they are on the computer screen.
10.4 In additive color, the overlapping of the three primary colors (RGB) produces a white light.
10.5 In subtractive color, the mixture of the three primary colors (CMY or blue, red, and yellow) produces a dark chromatic gray.
10.6 An illustration of three color gamuts. The visible spectrum is represented by the large, solid shape, RGB by the pink triangle, and CMYK by the area bound in yellow.
C R E AT I N G C O L O R S W I T H S L I D E R S
In both Illustrator and Photoshop, as in other programs, you have the option of working in RGB or CMYK modes. Choose CMYK. Your color training is with subtractive color and the CMYK sliders allow you to create colors in a way that mimics the mixing of pigments. Combining magenta (red) with cyan (blue), for example, will make violet. Also, as with paint, the addition of black (K) to any hue will make a shade of that hue.
Pushing sliders is, however, a somewhat abstract activity compared to the sensual experience of moving paint with a brush. Also, when using sliders, a few color functions are idiosyncratic. For example, on the monitor, colors are “tinted” not by adding white, but by moving the sliders to the left. The effect is similar to a mixed tint – a color becomes both lighter and duller – but the reasoning is different. Actually, the subtle and not-so-subtle differences you will encounter between mixing color by hand and making color on the computer will force you to process the structure of color in a fresh way. Students who follow up hand-made assignments with digital versions of those assignments strengthen their grasp of color principles.
Regarding Sliders in Terms of Hue, Value, and Saturation Hue
In CMYK, as with pigments, the only one-part colors are the primaries. All other hues are made by combining two or three of the primary colors.
Value
With any color, changes in value are determined by moving sliders to the left or right: Move to the left to lighten and to the right to darken a color. If you move all sliders involved in a color simultaneously to precisely the same degree, you will alter its value without changing its hue.
Saturation
In working digitally, continue to think of saturation on three levels: prismatic color, muted color, and chromatic gray. The numbers that appear in the boxes to the right of the sliders indicate the saturation level of the color on each slider. (The exception is black, where the numbers indicate relative value, not saturation; black has no saturation.)
When using a single slider – say, cyan – moving the slider to the right will do two things: gradually darken the value of the color and strengthen its saturation. When you have pushed the slider all the way to the right, 100% of the saturation of that color is achieved.
With two-part colors (not considering black a color), the same holds true. Moving the two sliders to the right will increase the color’s saturation. Three-part colors are more complex. A third part, if it is pushed far enough to the right, will diminish the saturation level of the composite color.