What is the green +20% affinity on a dragon in Dragonfire?

Every dragon has an affinity per troop type. For the type your army fields:

• Green (positive): +20% to the dragon's stats. Huge, and free.
• Neutral: no effect (the screen shows "No bonus").
• Red (negative): a penalty to the dragon's stats.

The golden rule: put dragons that are GREEN for the type you field into that army. If your army plays Spearmen, fill it with Spearman-green dragons. A neutral dragon works but loses the +20%; a red dragon is flatly handicapped.

It's one of the biggest controllable levers in the game — far above the ±7% matchup. You see it directly on the Army Builder screen: each dragon shows its bonus for the selected type.