I think there can be two ways in which we can be wrong about a colour. One is if we subsequently realise that we were experiencing an optical illusion. The other is if other people convince us that we were experiencing an illusion.
In both cases what matters is coherence - a yardstick by which we can establish objective truth. Other people can only convince us that we were mistaken if their account is more coherent than our initial account.
So objective truth for me does not depend on the existence of an external world at all, but on the coherence of our views about the world.