For eventing dressage(and most of the rules are the same) I use
-White Shirt and breeches
-Black tall boots
-Black leather gloves
-White pad with black piping(VERY nice looking)
YES breastplates are the only 'gadget' allowed. No whips, spurs are allowed(check the rules to make sure yours are legal, I use a prince of whals and it is legal at BN level eventing) no boots!!!
PS- this is my little fashion rant! Black can be just as unflatering as white. It is all about the way it fits your body! So, don't worry about a light color making you look bigger, it doesn't. If it doesn't fit you correctly, it will make you look bigger. ;)