So there's never been an official statement about this and it is in some ways contradicted by other bits of canon. So there are two explanations, and I will lead with the one I think is the most "real" first.

1, which is the out of character answer: It's just a stylistic thing.

A big thing about Paradox Live's art is that the official art (directed by Planarico) takes stylistic choices by the designing artists and canonizes them; you can see this most notably in how cozmez's white pupils and Iori's red pupils are directly lifted from (respectively) Kinako and Harada's stylistic choices in the original character designs. Aki Akane drew Hajun's hair very pale in the cover art for the Exhibition Show CD, and the art team probably just liked it and canonized it into the baseline style going forward (even though Aki Akane's later art for anniversaries has used Hajun's original more yellow hair color, but that art is less "canon" than the Exhib show art).
It also may be to help differentiate him visually from Dongha, though that is an afterthought since there was a full year between Exhib show and Dongha's introduction.

2, which is the explanation some people use to explain it in-universe: I've seen some people say that Hajun's phantometal erosion episode may have lightened his hair.
This is not fully substantiated by canon (Itsuki still has dark hair despite prolonged exposure to phantometal and Haruomi's hair going white is because he was visibly aged), but if you really need an in-character explanation, you can use that as a headcanon, since phantometal's effects are pretty flexible and as-the-plot-demands.