Every gem has a unique range of colors. Generally speaking, the purest and most vivid color a gem can have will also be the most expensive. But to really explain gem color, we need to look at the three factors that define any color.
The first is hue, which is what we normally regard as color: for example, a gem's hue will be red, orange, yellow, green, blue, or purple, or it might be in-between two hues: an orangey red or a reddish orange. For most gem varieties, a pure hue is most favored. Gems with a pure red, blue, or green hue are generally the most favored of all.
The second factor is saturation, the intensity of the color. This is the tricky one to picture. Colors that lack intensity often appear faded or washed out or look as though the hue is mixed with gray or brown. A fire engine red is highly saturated, a brick red less so. In gemstone color, more saturation is always better.
The third factor is tone, the lightness of darkness of the color. Pink has a light tone and maroon has a dark tone. In gems, a medium tone, not too light but not too dark, is considered best. Too light, and a gem's color is too pale to be attractive. Too dark, and a gem isn't able to sparkle with light.
A gem's color is evaluated for hue, saturation and tone. For example, the best rubies have a vivid pure red color in a medium tone, with no modifying purple, orange, or brown.
But each gem variety is judged on its own terms. For example, aquamarine always has a fairly light tone but its lightness is part of its watery appeal. Gems like orange-pink Padparadscha sapphire or blue green Paraiba tourmaline are so beautiful, they make gem dealers forget that pure hues are supposed to be better than mixed ones. These gems sell for premium prices simply because almost everyone finds them beautiful. So feel free to follow your own taste.