I almost feel cruel doing this, but in chapter 1, two pages after Himawari is shown in color with light hair, Tsubaki lists her "kogecha-iro" hair as something that captivates him. "Kogecha" (lit. "burned tea") being a traditional way to describe browns that are very dark, maybe olive.
I considered the color panel. It certainly could still be a coloring error, perhaps they simply applied a black and white filter to the cell phone photo, and that's it.
It might also be to save coloring time and other things that could explain why they leave the hair white instead of painting it, since the protagonist says her hair has a dark color it should be painted grey at least but it looks almost as if she were blonde or some light color that in the way manga is worked, it is always left unpainted. Doesn't make sense in the manga but it can make sense if we get into the production aspects of the manga itself.
But my theory assumes that this is an intentional decision to tell something more. We're in a manga where things aren't always what they seem, and reality can be altered. It's possible that Himawari herself has even altered how she's perceived, or how others are perceived, to fulfill her whims. Therefore, we, the viewers, are the only ones who can see the inconsistencies in reality that Himawari is altering. It's quite far-fetched, but everything that's happening in this manga is already quite far-fetched, so yeah, that's my theory.
But yeah it could simply be a coloring choice, but hey, that's what theories are for.