Hunter Thompson quotes it as being blue-and-yellow. I've also seen it described as having appeared on a studio poster in red-and-white. Jack Nicholson wore black-and-white striped trousers in one of his "biker" roles, which are probably intended to be taken as prison trousers ( do you get released from American prisons in your prison clothes? )
Now, blue-and-yellow would be typical for something intended to appear as black-and-white in a monochrome film - nurses' uniforms from "Emergency Ward 10" were actually brown-and -ellow, for example.. Re-and-white would be a typical "colourisation" of a monochrome print bearing no necessary resemblance to the original colours - there is a pic from "The Wild One" featuring Bruce Dern and Peter Fonda which has been colourised with red and yellow tints, knocking around the internet.
Interestingly enough, one of the other riders in that picture is wearing a black-and-white sleeveless t-shirt or crew shirt, or at least it LOOKS black-and-white in monochrome.. which is where we came in...