User blog comment:Xouzoura/What colour of haki do you think Luffy will decide to improve the most ?/@comment-1595565-20100830001011

I think it will be the color of observation. Luffy is often more oblivious than he is aware of anything at all, so the observation haki should balance him out. Then he'll master haoushoku haki, so then Boa Hancock really will fall unconscious when he looks at her, lol.