The Color pages are a set of modifier symbols for use in Ages in Mystcraft. They are used to affect the color of some element of the game. Some Visual symbols use them directly, while other symbols use them by way of Gradients.
When reading a new Descriptive Book, the Grammar keeps a "running value" for the current color. The first Color symbol in a phrase will set the current color, then when a new Color symbol is encountered, it will combine pairwise (averaging the RGB values) with the current color, to produce the new current color. This lets a writer specify colors other than those given by the 16 color pages. After the current color is "accepted" by a different symbol, such as a Gradient, there will (again) be no current color until another Color symbol sets it.
In effect, the combination "red blue" in Mystcraft symbols is like "reddish blue" (aka "Purple") in English. If "red, blue, white" is written, the current color will go from red to purple, then a light purple. However, "red, white, blue" will go from red to pink to a somewhat darker and bluer purple. Colors can be repeated, so "blue, white, white, white" will produce a very pale blue.