When artists say they repaint the horse when doing an art piece, do they literally colour the whole horse one colour then add the lines / muscle back on, eye etc?

Mostly yes, though they usually start with a few base lights and darks over 1 color, but it varies person to person :)

Yep! That's what I do.

Thank you both, interesting! Sounds like I should give it a try sometime :3

bingo! though i work in sections because a whole solid base scares me xD and i keep the original horse layer just in case+as a reference

Appreciate it's not exactly the same, but when I'm making digital art I fill my sketch in one solid light grey colour and then add lines/rough shading for the horse's muscles with a fairly dark/light grey. I do make them fairly thick (but not fuzzy) otherwise the horse looks stripey. Equally if they are too thick it looks a bit homogeneous after the next step. Then blend it together and add more mid-tones of grey. That just gives me a basic map for the muscles etc. Then add detail over the top.