After I posted with the titles "red and green should never be seen", which is the version of the saying I grew up with, I have had a couple of emails from people with the ones they know.
After much googling it seems there are lots of variations on this saying - I wonder why on earth people felt the need for them all?
Actually looking at them now they all seem to feature green- I wonder if it's just because more words rhyme with it?
But it also seems that not only are there variations on the saying, there are lots of different ending- you can mix and match to your pleasure!
Some I have found are:
Red and green should never be seen...
Blue and green should never be seen...
Pink and green should never be seen...
Some of the possible additions are:
...except on an Irish queen/ colleen
...without something inbetween
...without yellow inbetween
...except on fools and flowers (the only non-rhyming one?)
Other non-related sayinsg I found wee:
Never wear brown in town
Fawn should not be worn
Only wear yellow if you are mellow
I think the theory that these rules are just based on rhyming words does seem to fit.