There are a lot of times in Photoshop when we find ourselves working with one color space, but thinking about the changes in terms of another. As you'll see in Chapter 6, Image Adjustment Fundamentals, for instance, we often recommend that you use curves to adjust RGB values, but base your changes on the resulting CMYK percentages as displayed in the Info palette. So, it will speed up your work if you take some time to figure out how color spaces interactwhat happens in one space when you work in another.
Here are some ways to think about RGB, CMY, and HSB colors, and how they relate to each other.
One of the least understoodyet most importanteffects of adding colors together is that adding or removing primaries not only affects hue and saturation; it also affects tone. When you increase any RGB component to change the hueadding lightthe color gets lighter. The reverse is true with CMY because you're adding ink, making the color darker.
Every color, except the primaries, contains opposing primary colors. In RGB mode, red is "pure," but orange contains red alongside a good dose of green (and possibly some blue, too). In CMYK, magenta is pure, and red is notit contains some amount of yellow in addition to magenta. So to change a color's hue, you add or subtract primary colors.
In the process, you will probably affect the color's toneadding or removing light (or ink) so the color gets lighter or darker.
A saturated RGB color is made up of only one or two primaries; the third primary is always zero. When you add a trace of the third colorin order to change the hue slightly, for instanceyou desaturate the color.
Likewise, if you increase the saturation of a color using the Hue/Saturation dialog box (or any other), you're removing one of the primaries. If you get out to the edge, where one of the primaries is maxed out and the other two are still changing, you'll change the hue and the tone.
There's another important consideration pertaining to saturated colors. When you saturate a color in an RGB image, you wind up with detail in only one of the three channels. One of the others is always solid white, and the other is always solid black. It's thisnot just the difficulty of reproducing themthat makes saturated colors in images difficult to handle, because all the detail is being carried by only one channel.
A color made up of equal values of red, green, and blue is always a neutral gray (though you may have to do quite a bit of work to make it come out that way on screen oronce you've converted it to CMYKon press). The "darkness" of the gray depends on how much red, green, and blue there ismore light makes for a lighter gray. This is useful in a number of situations, including making monitor adjustments and correcting color casts.
For a quick summary of relationships among the color spaces, see the sidebar "Color Relationships at a Glance."