Catch 22, eh?
So you can't fly at night even if the flight is private, I assume?
Are you able to obtain an FAA medical which doesn't restrict you to day flying only? In that case, get such medical and find an FAA instructor that will endorse you for solo night flying. Find an N-reg aircraft and do the hours. Voila!
Rumours say that Australia isn't very strict regarding colour blindless but there aren't many Aussie registered aircraft/instructors in the UK as opposed to the US equivalent. Also, are you able to be sent solo night under the "UK NPPL" scheme? Those medical requirements are less strict.
Nevertheless, wouldn't the CAA be able to issue you with a CPL that is restricted to day only in the same way as the medical?