first of all im no pro, this is just one of the ways i color things, its hard for me to do a tut since i’m always changing the way i color but i’m gonna try my best at this, im also a terrible teacher
i use mostly sai and photoshop for textures or color changes bla bla, these are my tool settings on sai

it’s an approximate since most of the time i just move the bars around and mess shit up but thats usually what they look like, I use crayon tool to apply color and watercolor to blend it
so I usually start out by picking a background color and sketching on a layer on top of that

quick shitty sketch, doesnt need to be too accurate

Next i change the color of the sketch to the tone im gonna use to kind of shade in, idk how to explain it but it’s basically a guide of the hue that the picture is going to have its shades in. Example, I turn this sketch green, so all the shadows in the picture are gonna have a greenish hue to them, idk. For this particular picture I picked reddish orange, I set the sketch to multiply and lay my base colors underneath it in a sloppy manner

I merge the sketch and the color layers together and start picking out colors to shade in, laying them on with the crayon tool and blending them with watercolor. The reason i turned the sketch a certain color is, where the most saturated parts of the picture will be, I just pick a color from a shallow part of the sketch and the base color and use that tone to shade in. So yea just basically shadin shit around

here i added blue cause i thought it’d look cool but i was wrong and it ended up lookin like fuq and was a pain to get rid of but hey i shaded an eye so yeh skip this step, i just thought you should see how bad i fuck up these things

shaded the other eye and got rid of more blue bc hey it looked like shit

i start shading in the rest of the face, it’s also a good idea to bring some of the colors of the picture together, for example if his shirt is green try to bring in some colors from the skin in maybe the darker areas, or areas closer to his skin and viceversa, same goes for the hair

i put glasses on him on a separate layer, because i need to put the shade of the glasses on his face and im a pussy and dont wanna ruin his face so far.

then i toned down his colors bc my eyes were burning, with like hue/saturation or
whatee ver

at this step im pretty much done, what i did for the glasses was I put a layer in between his face and the frames, set it to addition, and put down a light yellow in the areas your light is coming from, blend that out with the water color tool and erase little shapes into them

now i went on photoshop and applied a filter that ruined everything so you should probably skip this step, no seriously idek wtf was going on but its done now so yea
careful with those and i hope this helped even though it probably didnt but i tried!
if you need specific help with something dont be afraid to drop me a message! I love to help out people and shiet