Are there any photos from previous shows you can look at to see what everyone else is wearing? That should give you a good starting point.
Traditionally, the discipline you show in dictates the colors you wear.
I know down here in the Southeast, in the hunter shows we wear beige-green breeches (Not green -- it's actually a pukey beige color), a white or pinstriped ratcatcher, dark blue jacket and black boots with black helmet. For jumpers, it would be white breeches with a black jacket, white shirt and black boots, black helmet.
And for classes that don't require a jacket (due to heat, etc.), a nice polo shirt will work.
Have you asked your trainer? If you're showing locals, there's a good chance that so long as you turn out properly dressed in a jacket and breeches, they don't mind color variations, so long as you don't mix and match wild colors. :)