Personally I don't think this holds any truth, I have a solid black and a solid gold both show girls. My golden is only 14 wks so is in the cockerdile stage but no more that any other pup of this age.
My black is 15mnths and is the gentlest, most laid back, cuddliest cocker ever (so much so that I think someone forgot to tell her she is a cocker) and has been this way since we got her at 8 weeks. She is also extremely good with the pup and lets her get away with everything.
I think that this is a myth and as long as a dog is bred for temperament by good breeders using good health tested dogs then colour is irrelevant.