I agree with Vicky that it's probably okay to wear a dress that's the same as the wedding colors. Neutral colors like chocolate have surely been worn to weddings with the same color scheme before. Just make sure you don't look like a bridesmaid!
If you know the wedding is, say, chocolate and turquoise, then accessorize with any color other than turquoise. Or see if the chocolate colored dress you want is available in a print that uses chocolate and another color.
If you're confident you're looking your best and not self-conscious about the wedding colors, then probably no one will notice or care.
Now, I question chocolate as an August wedding color because I'm picturing lots of melting stickiness, but that's neither here nor there.