The colors are always relative to you...so you will always look green to yourself. So will any players or Animated who are within 0.5 Charge Level of you.
Players, and Animated, who are weaker than you but still within 1.5 Charge Level will be blue. If they're weaker than that they're purple.
Players, and Animated, who are stronger than you but still within 1.5 Charge Level will be yellow. if they're stronger than that they're red.
Remember that these colors also apply to the star icons that indicate Animated on the next screen.
Also, the colors affect your chances of getting loot, including Gold and Charge Orbs, from defeated Animated. Blue Animated drop less than Green do, and Purple hardly ever drop anything. If you want loot from an area that you're too strong for, you'll need to suppress your level to get anything. On the other hand, Red Animated drop stuff less often, too! The devs didn't want to encourage you to charge into battles you can't handle and get yourself slaughtered in the name of loot.