Alright, here's a super short, basic tutorial on coloring and lineart. Warning: I'm not a professional, so my explanations are probably gonna be choppy.
I'll be drawing a headshot of my Nightwing OC, Judah.
1) Sketch
Yep, just a sketch. As any sketch, try not to make it too detailed. For me, it helps when the background is a light or dark shade of grey. A lot of professional I've seen tend to use grey canvases. I'm not exactly sure why, for me it works better than a white background.
Focus on the shapes. The roundness of the head, triangle horns, cylinder neck... all that.
2) Lineart
Once you're satisfied with your sketch, you can begin to do the official lineart. But this doesn't always have to be the case. If you're happy with how the sketch looked, work with the sketch!
A tip with lineart is to not add too much detail. Save that for the coloring and shading. Too detailed-heavy lineart could make the image look messy and complicated.
3) Color
I put down the most noticeable colors of my character. Not too many small details, just the overall colors of his scales, eyes, and underbelly.
Notice how I didn't fill it in completely? In my process, I sometimes like to leave small lines of white or grey to indicate highlights. For me, this helps highlight structural features on the face or body.
Let me pull up a quick example from one of my inspirations, Remarin. They use this technique multiple times in their works.
Notice how the abdominal muscles are highlighted with a pinkish line? It makes the features "pop" more than a flat, dark surface.
4) Shading
Alright, this is probably the most cliche part. Mostly because it went from plain colors to (poof) almost completed piece. That's because, what I do, is I blend with the colors I have.
Never use complete black for shading, always use a darker color of what you have. If your character is pitch black, I'd suggest staying away from totally dark settings, or at least adding some different colored light.
As for the eyes, I use a soft brush to make a lighter version of the eye color, then put the pupil on top as a much darker version. To top it off, I make a dark shadow on the top, lower the opacity, and add two highlight dots on the top and bottom.
One of my other inspirations, Reykat, does this in her pieces.
5) Highlights
Another technique I learned from Reykat: adding highlights. I might have put too much on this one, though.
This step helps make the piece look more "alive" and not just a flat image. Some spots to put highlights are around the eye, on spikes or horns, alongside the snout/back, etc. ,
Hope this helps! If you guys have any questions let me know :^)