Hue = Basic Color
Take another look at Figure 10.1 and you'll notice that only six basic colors are shown: cyan, blue, magenta, red, yellow, and green. That's because every color you could ever imagine is based on one of those colors or what you get in the transition between them. Take red, for example. Darken it and you get maroon, or make it less vivid and you'll have pink. But in the end both are just different versions of red.
The basic color that any color is based on is known as its hue. Photoshop describes these basic colors, or hues, using numbers that it gets by figuring out how many degrees the color is from red going clockwise around a color wheel. If you divide the color wheel into sixths and start with red at 0, then you'll find the other colors as follows: yellow at 60°, green at 120°, cyan at 180°, blue at 240°, and magenta at 300° (Figure 10.2). You don't have to remember any of those numbers, but it will be helpful to know that hue numbers in Photoshop are based on the color wheel. When you adjust the hue (using an adjustment like Hue/Saturation), you're effectively spinning the color wheel by moving each basic color in your image an equal amount (or angle) around the edge of the color wheel.
Figure 10.2. If you divide the color wheel into six equal parts, you'll find the primary colors that make up your image (RGB, CYM).
The other way you can shift the basic colors in your image is to push them toward one of the six primary colors that are found in the color wheel (using an adjustment like Color Balance). Red, green, and blue are the exact opposites of cyan, magenta, and yellow. Cyan ink's sole job in life is to absorb red light, magenta ink's job is to absorb green light, and yellow ink absorbs blue light. That's why you'll never find an adjustment that allows you to shift something toward cyan and red at the same time. They are opposites, so moving toward red automatically takes you away from cyan. When you push your image toward one of the primary colors, all the colors within the image shift in that direction and become more similar, whereas shifting the hue by spinning the color wheel leaves colors as different as they used to be and moves each one an equal distance around the color wheel.