Exactly The Blue ones run out first more popular don't you see, any way you can get a green one and paint it blue, simples
Green ones lasted a long time, though - I remember seeing 075 in the late 70s, although I have to say that I am not sure if was green any longer, as it was mostly dirty brown!
(There was blue on the cab side - under the number and also, of course, under the TOPS data panel.)
For those with an interest in such things, it had worked down from Nottinghamshire via Market Harborough to Northampton, where it came off the coal train (for Didcot, via Bletchley and Oxford, I think) in exchange for a WR 47. In years before that, it could have been a Wessie, although I never actually saw one myself.