Fighters, zombies, and all enemies have colors. Each color is strong against one other color and weak against one other color.
The strengths and weaknesses of each color are given below.
-Red is strong against green and weak against blue
-Green is strong against blue and weak against red
-Blue is strong against red and weak against green
-Yellow is strong against purple and weak against itself
-Purple is strong against yellow and weak against itself
Normal attacks and damage done by matching tiles is doubled against targets your color is strong against, but only does half damage against the color it is weak against. Whether fighting players or monsters, the different between using the right and wrong color is 4x more (or less) damage!
Check the relative strengths and weaknesses of colors in game by looking at the chart in the upper left screen during battle.