I've heard two versions of "crabs" for the RAF
The first is that the colour of the uniform is (or was) reminiscent of the colour of some of the inedible bits of a crab (don't ask me, I can't eat crabs etc. so I don't go near them). RAF blue is known in some circles as "Crab fat blue".
The second version is that the original shade of RAF blue was similar to that of "606" or "Salvorsan" (also known as "blue unction") an ointment for the treatment of crab lice.

DonD, Eric Partridge records the existance of rhyming slang both in Australia and the USA (with a subset for tramps/hoboes), if I can find a reference, I'll post it over the weekend.

Walrus