I had a black metal one until I decided it needed a close encounter with a spray can of B&Q fluorescent green paint. 3 years later I have a green one with black patches, but still enough green to be visible from a distance.
You could try this guy who trades on ebay. He has a range of colours, and says if you don't see what you want to contact him, so could probably source one for you
Many thanks. Yes, Norfloat have told me they can do them ... and at an attractive price. If their product and delivery are half as good as their response to e-mails, we're on to a winner.