Let there be colour!
Wedding cakes do not need to be white! There is no rule that says this anywhere. Add some colour to your cake! Your wedding cake is part of your reception decor - design it to compliment the theme/style you have planned.
So many pretty colours in the rainbow out there - use them!
It is your wedding, and you should set your standards. If you want, you can even go with darker colours like plum, purple or black.
Keep in mind that with darker colours - definitely fondant is the way to go for the exterior of the cake. Deep dark buttercream colours will often leave guests lips a funny colour - so using fondant for the design will help stop that from happening.