Elm: That's a lot of pixels. Like, 9 times the number of pixels found in Syosa's work (example:
A writer and a Dragon : 201x225 pixels = 45,225 px, vs yours 500x800 = 400,000 px.). If you want to take an oekaki-style approach, no problem, but if you're aiming to work on pixel technique specifically , I strongly recommend reducing the number of pixels you'll need to work over. Right now, it's more like an illustration (nice hair, btw)
Regarding picking your own palette: I'd suggest looking into MyPaint. Its HCY color selector is much more intuitive than your average HSL/HSV/RGB selector, and it also includes gamut masking, which is a really nice way of
simplifying the problem of creating a unified palette.