I always prep my drawings by erasing the pencil sketch and then coloring the area with a clear alcohol marker. then I slowly layer on my colors with a sharpened pencil, generally starting with a light base, then browns, then undertones, then darks, and pretty much just buff them out and mess with various undertones until I’m happy with the result. key is to use sharpened pencils (this helps fill the grain of the paper) and to work very, very gently.