As far as I know, and I've had a few caravans and so have friends, they are not colour coded you just match the colour with auto paint. If they were colour coded it would be stamped on the weight plate but isn't. If it's metal fine if its plastic/fibreglass seek advicve from manufacturer on how to paint it.
This is their address and phone number if they haven't moved.
Elddis Caravans
Delves Lane
Consett
Durham
DH8 7LG
01207 503477
bb
March 2011