red = hot = danger.I like the Incinerator Trooper. It's quite a striking colour scheme on display, and the colours make sense: red = hot = danger.
The yellow for the Artillery Trooper didn't make that kind of innate sense, and neither did the idea of the mortar itself.
As for the use of colours themselves, it makes sense visually so commanders can spot the positions of their unique troop types in a combat situation.
yellow = mustard = hot dog