A tutorial?
From me!?

I'm not really sure I should call this a tutorial.
But using the image above as a reference, I put together a cel coloring guide that you might be interested in^^

1> The first image is obviously the line art. Cel artworks are, from my observation, have generally thin lines. Unless it's something like 70s anime "sketchy" style (used on most Go Nagai anime, and recently Gurren Lagann), you don't need too much line weight variation and shading you would normally do if you're drawing manga (which their quality are really dependent on your inking). The line art is there to "contain" the colors, so you don't want them competing too much with your paint.

2> Next, I make 2 layers with single colors showing a plan on how I'm going to apply shading and highlights. By using single colors, you would focus more on planning how the light source would hit your figure. I find this more efficient than deciding the lighting by how you shade the first body part you decide to color. Also note that you don't need to strictly follow the guide (I know I didn't) but it's better that you made the guide anyway.

3> And now that I have the highlights and shading guide done, I create a new layer containing the first layer of shading - that is the basic shading that shows which parts of the model are hit by light, and which aren't.

4> I create a new layer for the second layer of shading. That is, applying a darker shade of color on certain areas that are hit by too little amount of light. Also, I use this layer to apply core shadows (using the same shade of color). Just in case you don't know, core shadows are the dark areas in between the part that is hit by light, and the part that's not hit by light. You still can see the part that's not hit by light because it is hit by reflected light. The core shadow, in comparison with the area hit by reflected light, is a little darker. Try observing stuff from real life to see what I mean.

5> Now with the shadings done, I'll hide those layers, and make a new layer containing the base colors. I apply shadings first because there are times when you would use a base color that's very light. These light colors are more visible when they're juxtaposed (ooooh juxtapose) with darker shades of color. It just makes coloring a bit easier for me, but if you feel you want to apply base colors first, it's up to you.

6> Now I make a new layer, place it below the shadings layer, and apply the first layer of highlights. these highlights can be very light versions of your base color, or they could be the color of the light source blending with your base color. Keep in mind that surfaces absorb light and reflects them back as color.

Also, pointed out to me that the edges/corners of objects are a little brighter because of reflected light.

7> Create a new layer above the first highlight to apply white highlights. They are basically the brightest part of he highlight, which is why they're white. Use them in moderation though. I made a couple of erasures because I'm white highlight happy.

8> And finally, do whatever you wish with the image. You could make a third highlight layer and use a large soft brush to manually add glows on some of the highlights. And from what you can see, I kinda overdid it.... again T_T

The artwork I used is from an unused planned illustration showing Astral and his mother-complexed counterpart which I can't go into more detail about just yet. I didn't finish the illustration because I kind didn't like how Astral's gun turned out.