Yeah the title says it all.
Every hair color doesn't need qi. But pure does (as you can see here).
Why?

I'd like yo buy one when I get hair but I must have 10k qi :/

I think there isn't a particular reason, they just made some changes thats all

pure hair color doesnt change ur hair color im not sure if its a problem or is meant to be like that

I believe the idea is that only old people have gray/white hair. So you need qi (Years of experience) to be an oldie.

Well other pure items need a hi qi too, I think this is a possible reason too

Hmm I didn't know about this.


Well, I don't think so.
If that was the reason, void, demon and elf should require qi too, right?

Guys, just letting you know, ^all that is offtopic.

Anyways, I would have to say that is is probably the most wanted hair. I mean, white hair could go well with any set.
Now you are probably going to be saying, "Oh what about void and demon?".
Well, I don't know.

Does this rule works only to pure ?

Because pure its the best :P

pure has always needed 10k Qi.
Kept as a reward to those that stick around that long.
although it is odd that the others dont have the usual Qi reqtrictions