Where do the troops I put on my dragons come from?

Troops are generic: they come out of your castle's Barracks, not from any particular dragon. You build a stock of each type (Cavalry, Shieldbearers, Archers, Spearmen, Siege), then assign one type to each army.

What that implies:

• Any dragon can carry any type — but its damage changes with its affinity for that type (+20% if green, nothing if neutral, a penalty if red).
• Produce your main type first (the one your combat armies field), not a little of everything.
• The volume of troops on each dragon is both your fuel and your punch: the more you load, the harder the dragon hits and the longer it holds.

So: produce plenty of your chosen type, and fill every dragon to the max before you set out.