As far as for the rules, you're going to have to check what's legal in your area. In Canada any show sanctioned by the EC beyond a "discovery" rating requires full dress, including tall boots.
Colour is frowned upon, and is usually illegal. In Canada what is allowed is a dark coloured coat, light coloured breeches, stock tie and gloves with a black or navy hunt cap, bowler, top hat or protective helmet. You are also only allowed neutral coloured bands in your horse's mane, ie black, white, grey, brown. Personally, I always use bands that match the colour of my horse's mane. Your boots can be of any colour (as far as I know) except in championships and FEI levels.