I was wondering as to why some Esperanto colours are not just the root with an 'a' suffix attached. These are: rozkolora, violkolora and oranĝkolora. Why not, roza, oranĝa and viola? Or can you say this?

Probably because oranĝo, rozo, and violo are all existing objects: orange (fruit), rose, and violet (flower), respectively, so changing them to adjectives results in other meanings, e.g. "oranĝa" means "made of oranges" and "roza" means "rosy".

You certainly can use oranĝa, roza, viola as colour adjectives.

But if you refer to an oranĝa arbo, when you mean an orange coloured tree, expect to be misunderstood. On the other hand a 'salamandro kun nigraj kaj oranĝaj makuloj' would be immediately understood to refer to the colours of the markings.

As a previous poster pointed out, a difference between say ruĝa, verda and oranĝa, roza is what happens when they become nouns (ie you change the ending to 'o').

Yet you may still be able to preserve the idea of colour if context makes it clear - e.g. la rozo de ŝiaj vangoj.