It's just a lot of experimentation, really! I use windsor newton acrylics and satin glazing medium, usually I'll do a plain base coat (kind of like cel shading) and then glaze different colors on top of that with very thinned down paint with medium. Something I like to do is paint lighter areas with transparent or opaque white and then glaze over them for colored highlights (works for shadows too). You can even do the value of a painting in black and white and do the color on top purely with glazes, you'd be surprised how well it does.
Usually it's thin enough that I can use just the medium and paint, but if it's more paint and medium, I thin it with a little water. Medium takes getting used to, but it's worth it!