Ever since my daughter was old enough to pick out her own clothes, she's deliberately chosen things that clash. She came home from kindergarten and asked me why people wear a plain t-shirt and jeans, and she really was confused about it. She once rejected an outfit of blue plaid pants and a striped top because they "matched." I challenged that, and she said that there was a little blue in the shirt, so it wasn't clashy enough.
She just loves color, and I totally encourage it. When she was about 4, one of our neighbors once tried to help her by suggesting that when she wears a patterned top she should wear plain bottoms, and vice-versa. My daughter's response to her? "Why?"
Well, because that's what you're SUPPOSED to do.
Which brings me to wedding cakes. A lot of people bring me pictures of cakes that they've cut out of magazines, cakes that have a lot of color on them and are completely nontraditional. After they think about it, though, they (or someone they've brought with them to the tasting) talk themselves out of it, and end up with a white cake that might have some color on it, but nothing too "out there."
They might want a colorful cake, but that's not what you're SUPPOSED to do for a wedding cake. So they end up getting something that they like okay, but it isn't what they really wanted.
Some of my favorite cakes that I've done have no white on them at all. Some have a lot of color and a little white, but all are designs that the couples wanted regardless of what you're SUPPOSED to do for a wedding cake.
If I sense that a client is at all open to doing something that isn't totally traditional, I try to encourage that. I'm not saying that I don't like traditional cakes, but when I get the chance to do something different I'm going to jump on it. It doesn't have to be colorful, either. There are plenty of options for an all-white cake that can personalize it and make it memorable.
I guarantee that cakes that are unique are the ones that people say "oh, that's so 'them' " and remember after the reception.
Don't be afraid of using color or nontraditional design just because it's a wedding cake. The cake is a centerpiece, and you want it to be something that you like and that represents you as a couple. If you want to use color, use color. If you want to use an unusual design, do that. If you want to keep it all-white do that, but make sure that you add something to the design that makes people realize it's your cake, not someone else's.