something something try choosing a color with a higher chromatic value for highlights vs "color lightened with white"

...and I misspelled 'highlights' whoops 🫠

Here's a better visual rep for what I mean wrt highlights:

I too still fall prey to that upper-left quad of the color wheel when it comes to highlights, but it can often lead to a more dull look- which if that's what you're going for, great!

healthy, shiny hair is typically more dimensional though

and my base sketch!