Thanks for that but I looked it up and it says under Schedule 17 part I Paragraph 7 Sub-paragraph (b) - A solo motor bicycle, a motor bicycle combination, a pedal cycle with or without a sidecar or an invalid carriage: No requirement
Now I'm not a lawyer so may be wrong but it looks like sub-paragraph (a) is for any vehicle not covered by sub-paragraph (b), and that requirement is for amber or if within 1m of the rear of the vehicle it may be red. So if motorbikes fall under sub-paragraph (b) it looks like we can put whatever colour we want on...except blue which seems for be for plod only.