To get the most use out of color stacking you need 2x to 3x rainbow teams which you are working on.
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Defense and Attack buffs
Generally speaking healing special skills and damage special skills are the best.
Extra attack is called an attack buff. Extra defense is called a defense buff.
Defense buffs are cool as a secondary effect ( Kiril, Wilbur, Aegir, Victor, etc. ) but has a limited duration and can be hard to time.
Attack buffs work better because you can time it easier.
One problem with color stacking, it does not help with your attacking team’s defense since enemies use tinted damage ( see link ).
So if you pick an hero of the correct color, with an attack buff, even dead they will do double tile / matching/ physical damage ( strong color stacking, grave damage. see above color stacking quote ).
While picking a hero of the correct color, with a defense buff, they might be killed before they can use their special skill.