How do you do the coloring markings? Like this:

Val, if you dont mind, I am using a piece by you ^_^.

https://i.ibb.co/Y07gxyz/thetis.jpg

Just like adding white markings? Or paint them.

I use the lasso tool to select the shapes I want (or I brush them on a separate layer for more detail and select the outline of the layer instead).

I then go to the layer of the horse, go to my adjustments and lower the saturation to almost 0 (not exactly 0, depending on the colour I want).

Lastly, I change my brightness/contrast and play around with my colour balance with the selection only.

These options are found in the toolbar at the top, under image > adjustments > etc.

Of course, blending and that will then depend on your style, smudging edges, etc.

Amanduh has some awesome walkthroughs to show basics and more <3

https://www.youtube.com/channel/UCBja60SdRTmGBaypsBgyowg