After speaking with the Cartos, it seems we've arrived at a consensus. What we would like to see is some sort of graphical theme at this point, though not a full blown watercolor. Theme needs to match the concept and period depicted to create a cohesive map. Many feel that continued use of the dreamy tufts of translucent color are not suitable for the theme of Colonial Africa.
That said, I don't think you won't be able to use watercolors as all. I much prefer the medium to photoshop colors and the sterility of computer perfection... but I think you should experiment with style a bit on this map to give it a theme that reflects the colonial world. Personally, my inclination is to go the military route and drive hard the symbols and stylings of 19th century political and military maps. But that's just what I'm thinking, hopefully it will jog some ideas for you.
There was also some mention of the map's gameplay resembling Eastern Hemisphere or South Africa 1885... for me, at least, this isn't a huge problem but I figure you ought to know how the others are feeling about the gameplay at this point. You might want to change some things around and experiment.