Well now, in this tutorial, I shall show you my awesometastic techniques and share with you some tips that have helped me through many difficult situations, such as when I got stuck in a doorway or when I couldn't choose whether to eat the blue M&M or the red one first. Now, young adventurer, prepare for a long journey, for it will not be easy.
Alright, first things first. Come up with an idea or pick a reference picture, and make the sketch. I suggest using low opacity (I used about 15) It can be neat (like mine, because I'm such a perfectionist), or messy. It doesn't matter in the end, because you'll be going over it on a different layer in black! :D For the sketch, I use the line tool, mostly because I can't make straight lines by myself with the mouse, and using the curvy lines takes too long. The curvy lines are, however, very helpful with the outline. But you can use whatever's easiest for you :3
So now, on a different layer (which I named "Skin" >_>), I have started to color, yes, his skin. In the picture you can see I have circled various parts...it's easy enough to figure out, but just in case: the blue one also indicates the area you should use so it doesn't turn out too yellow or too pink, and the red ones indicate where you should generally pick colors so they don't turn out too bright or too dark. Unless, of course, you're making a fantasy drawing. If so, then feel free to use whatever color you like.
Now onto the actual coloring. For my technique, you'll want to color with high opacity (I use full). Don't worry about being messy, you'll be able to fix that later.
Not much to say here. I chose a darker shade of the base color (note the little arrow. Note it.) and, with low opacity, started the shading lightly.
I forgot I had this picture, so >_> Here I've outlined in red the areas I shaded. Don't worry if it's too light, you'll be gradually adding more darker colors over it.
Again, not much to say. I chose a slightly darker color and, also with low opacity, shaded some more.
Same as above, just showing which color I used next.
Now here I started using a reddish color to shade. This is important: don't assume that to shade and highlight something you only need different shades of the same color. I usually use the two colors around the base color, for example, if I'm coloring grass, the base color will be green, I'll make highlights with yellow and do some shading with blue. Or in this case, the base color is a very light orange, so some of the shading will be with red, and some of the highlights will be with yellow (you'll see that soon ;D)
Apart from using just the colors around the base one, you can also use another one -lightly- for the shading. I say lightly, because if it's too obvious, it won't look natural anymore (having something else giving on a certain light and shining onto the subject is a different matter. See KakeranoTsuki's color theory tutorial [ LINK ]).
So in this drawing, I used a light grayish purple for small areas in the shading (which I have marked). When using a reference picture, really look at which colors it has, like if you're dissecting it (lolwut). And lastly, in general, using too bright colors to shade just doesn't look good if you're trying to make it realistic.
Phew, finally starting the highlighting. So I went back to the base color and chose a slightly lighter shade. I've marked the areas I highlighted.
Like in step 7, here I've started a light shade of yellow.
Now to accent the highlights, I used the lighten tool on the areas marked in the picture. If you're making a realistic drawing, it's important that you don't use it too much, and that you use very low opacity. Doing the complete opposite of that can, however, look fantastic if done correctly ;)
Now to finally start the hair (told you this was gonna be way too long). Make another layer, call it whatever you like, and choose the base color for the hair. As I've indicated in the picture, using brighter colors is more for cartoons. The "lighter colors" just indicates, well, the lighter colors, but those would also be more realistic. Since Zidane is technically kind of a cartoon, I strayed into the bright area. The blue circle just indicates which area you should use for blond hair.