Anime

fill it with different colors would do the trick...?

Perhaps use some dodge and burning...

Shading in the manga style would be using a very fine grain pattern all the way to a very broad lined pattern. And apparently, most of the time it's done in black in white. The only exceptions are the special ones where the first few pages colored or the anime-to-manga conversions.

If you want to do it in the anime style, the colors are mostly solid patches of color. Shading is usually using a single solid color or a subtle gradient, all in which are in definite shapes.