Reverse the color scheme and the colors would be nearly impossible to distinguish from Persia's in-game. :/
And as far as a good color scheme that hasn't been taken that has yellow as its background...? Er... the least awful would either be some form of blue, or white, and not sure that either one fits Siam. But, eh.
I whipped up a couple sample color schemes; they would look better if I actually remade the icon instead of just recolorizing the symbol, which I'll do when I actually make the mod.
Here's my opinion: I could switch Siam's symbol color to purple, which I think would look best, then change Egypt's to purple.