Why shouldn't I put a red-affinity dragon in an army?

Putting a red dragon in for the troop type your army fields is the costliest composition mistake there is. Red affinity reduces the dragon's stats: it hits softer and soaks worse, when you could have had +20% with a green dragon.

The gap between green and red on the same slot is enormous: you go from a +20% bonus to a penalty. That's a multiplier you're handing to your opponent.

The reflex before you confirm an army:

• Open the Army Builder, select the type.
• Check that each of your 3 dragons is green (or at worst neutral) for that type.
• No red. Ever. If your best dragon is red for that type, either change type or take it out of that army.