I don't think any of the colours are rare, to be honest; plenty of people got blue FGT's before the OCD was added, and then loads probably buy the ones that come up in the OCD too, so rarity isn't a consideration as far as I'm concerned.
When I've finally bought all the premiums I'll probably try and replace my blue one with another if a better one pops up in the UCD, I'd like a green and yellow, green and gold, black and gold, black and yellow or just black one if they come up.