I have many techniques for coloring but one that a lot of you probably noticed has been my cellshading used for knuffels. This is actually a fun and easy coloring style that you can do to.
Now, I generally recommend doing stuff in real media first. The real media equivalent of my cell shading is actually heavy inking. 
You have to really embrace shadows and adding a lot of them. However, if you aren't really keen on doing shadows this might be a good introduction to dramatic shadows.
The obvious first is the lineart.
At this point I add in the base colors
You get to add in a lot of the gradients and makeup in this stage.
This also allows you to see if the colors work well together.
Now on a new layer you are going to choose a nice dark color (in this case I chose a really really dark green) and you will put that green absolutely everywhere you believe a shadow might be. This is going to be a flat color and won't look overly great but trust me, this will make life easier later. This is also why its very similar to inking.
Because this is on its own layer you have a lot of freedom with it. If you want a lot of dramatic shadows you can just lighten the layer just a bit. (in this case about 80%)
If you aren't after as much drama you can instead lighten it up more (in this example about 20%)
I tend to prefer about the middle of the scale.
At this point I wanted to give the cloths a bit of a shiny to really define the fabric. You can also use this for shine on the eyes. Basically on a new layer you add a bit of flair.
Now for highlighting. At this point you can repeat what you did for the shadows but I prefer to use an airbrush technique to soften up the light a bit.
What you want to do is on a new layer get the lasso tool and select all the areas you want lighlights to appear. You are going to get a soft brush and put the coloring to low (like 12% or less) You want to lightly go over the areas you want to lighten up. Even sometimes a few times to get it about how you want it.
At this point you probably think you are done. However I like to add something else for added depth. An interesting fact about light is that in a lot of situations theres a backlight caused by a bounce back from the original light on other objects around the character. Also sometimes theres just a second lightsource in the area. So you can have fun with this and add more color to your picture.
For this I use the same technique for making the shadows however its a bright color.
And viola.
