EliteKeyboards used to sell certain coloured Topre (Trope?) keycaps, such as Enter, but had only a few colours left and no stock of those any more.
I don't know of anyone who sold the HHKB arrow keys in specific colours - you'd have to buy a full set for that (as Halverson said).
A while ago (maybe a year) someone was selling random single coloured Topre keycaps, so your best bet might be to create a WTB thread in classifieds and hope that someone somewhere has a few to sell.