it's very rare. but, i've seen three, and have gotten two. :d the first one was a pure white HM with massive fins. i was gonna get him, but by the time i got the money, he was gone. >3> the second was my avatar betta, a white HMPK named weiss, but you can see in the pic, he developed a black mustache, and light pink in his tail and anal fin.
my current is another white HMPK, a blind boy named Theodore. he has a tiny bit of black on the top of his ventrils, but it's un-seeable usually, and if he ever flares(which is VERY rare), there's a thin line of black on his beard. very thin.