How do I know which dragons are green for a troop type?

Open the Army Builder screen, pick the troop type, and read the bonus shown per dragon: green = +20%, "No bonus" = neutral, a penalty = red.

To build a single-type army: list the dragons that are green for that type, then take the best 3 (keeping a Champion or Warrior for the Center).

A sample green spread (a typical roster):

• Spearmen: Crimson, Vaeldra, Jagadrix, Caraxes, Rhysarion, Syrax, Bevlorin, Dawnseeker, Venator (9 green).
• Archers: Crimson, Seasmoke, Zivern, Daemoros, Tashix, Solstryker, Syrax, Arrax, Antares, Vhagar.
• Cavalry: Seasmoke, Kalspire, Caraxes, Feskar, Malachite, Shadowsong, Thunderstrike, Shimmer, Arulix.
• Shieldbearers: Kalspire, Malachite, Rhysarion, Velar, Arrax, Shadowrend, Nyrena, Vesper, Venator, Vhagar.

The type with the most of your good dragons in green is usually your best army axis.