User blog comment:Xouzoura/What colour of haki do you think Luffy will decide to improve the most ?/@comment-1705954-20100831024600

Rayleigh did not tell him that he would let him decide what color he was best at. That was a mistranslation spread by Binktopia. What Rayleigh actually said was that while most people stick with the haki type they are best at, it is more advantageous to train in all of them so that it was he is going to do with Luffy.