See also: palette swapping

This is common in MMOs but usually limited to the ever present name tags that float above every critter you encounter.

Older games with miniscule graphics (in terms of used pixels) often resorted to palette switches for sprites, which usually matched how much tougher the individual variant was. However, this does NOT require palette swapping to be involved. The colored enemies can be wholly unique, they just have distinct coloring to show other traits.