What happens if I put a red-affinity dragon in an army in Dragonfire?

A big trap. Dragon↔troop-type affinity has three states:

• Green = +20% to the dragon's stats (plus a siege damage boost).
• Neutral = 0 (no bonus, no malus).
• Red = a penalty (the dragon loses stats).

Putting a red dragon on your army's type is deliberately weakening it. A lost +20% plus a penalty on top weighs far more than the ±7% troop matchup.

The hierarchy to remember: you build an army around ONE troop type, and you only put green dragons in it (or neutral when it's painless). Never red.

Don't confuse them: the +20% affinity (the dragon's stats, per dragon in the formation, shown as "No bonus" when neutral) has nothing to do with the ±7% troop counter triangle. The first is huge and controllable, the second is minor.