Strong damage is double normal damage. Stacking also increases the damage, a little more than linear.
Using double heroes of the same colour on defence encourages stacking against that colour and doubles its effectiveness as it is strong against multiple heroes. This also eliminates one colour that the attacker will be weak against. For instance, if you have 2x blue and no green on your defence the opponent can stack green and blue, green to be strong against the two blues and blue to have no weaknesses.
Using rainbow minimizes the stacking effectiveness as each stacked colour is only effective against one hero, and is weak against one as well.