Bryan, you are 100% correct about red fire engines except........ airport fire engines are in the main painted yellow, this must be to make them go faster when travelling up the runway.
Dave, no no no. It stems from forces slang really. The RAF regiment is known as Crabfat Crabs go sideways and fat is yellow!" Yellow as in chicken. 
No, just a historic word spelling change I suspect. The original was CrabFAST (assuming yellow involved).
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