I was a bit uncertain if the girl on the left really was Kasumi, because the colour is a bit off, but her hairpin checks with her 1st outfit, and her hairstyle is the same as well. I think the artist is just colourblind. Lmao

ManuBonito said:

I was a bit uncertain if the girl on the left really was Kasumi, because the colour is a bit off, but her hairpin checks with her 1st outfit, and her hairstyle is the same as well. I think the artist is just colourblind. Lmao

So it seems that when this was first posted to Twitter in January 2021 it was alongside an another animation of Kasumi with the same blue hair.

The artist also posted a very similar animation to their Pixiv (post #8056687) of the same scene of Kasumi banging a pot to call folks to dinner. This time it's explicitly tagged as Kasumi and has her typical grey hair. It was posted later in August of 2021, and I'm not 100% certain, but it appears to probably be a repost from 2015 (I can't find the original posting if it was, but many of the artist's prior sources are now dead).

Other older posts of Kasumi by this artist (e.g. post #2197117 and post #2135177) also feature grey hair.

So it's almost certainly the case that it was just a conscious artistic decision to tweak her hair color to be somewhat bluer.