Regarding the colours: the map looks fine through both colourblind filters, and I haven't heard of any specific problems about them. So I'm inclined to think the colours are ok now unless someone can show there are two specific colours which can't be distinguished from each other.
As for gameplay, I think it's fairly balanced now. I don't think Goguryeo is overpowered. Yes, you can get a high bonus on it, but it requires you to grab a large swath of land. It's harder to defend than either Silla or Tang. If one player holds Goguryeo and the other holds the equivalent amount of Silla + Tang territories, I think the one with Silla & Tang has a much stronger, easier to defend position.
As for drops, the drops are coded to be fairly balanced. No one can drop with any bonus, and everyone gets to drop next to capitals.
For that matter, I'd suggest Danryan to play a few games on it before speculating on the gameplay. As a mapmaker I know how hard it is to analyze the gameplay of a map without playing it, it's (partly) the reason we have beta maps. I'm willing to wait for further input, but at the moment I don't see any issues with the gameplay.