Hmm.. I should make a tutorial on this sometime..
First, the brush.
The first to know before using the brushes for painting in photoshop is to know what settings to use to maintain control over your coloring.
A good thumb-rule is that when ever using a brush your should always draw with a low opacity, but even more important is the flow option. I normally work with a flow around 4-7%.
Now, the regular plain brush that you'll use for most of the time when coloring is the smooth, soft, airbrush-looking brush (low-to-none hardness and in the "brush presets menu" set spacing to 3-7%).
I always find this the easiest brush to draw with.
Another brush that is a bit harder to control is the hard brush (hardness 100%), this even makes it look more like a real brush.
Another version is the "Pencil Brush". This is a more advanced brush, that resembles real brushes even more. Theese can be made by adding textures.
Another trick is (if you do not own a Wacom Tablet) to use "fade" in the "brush presets menu".
The first I always do before painting is determine then ammount of light and what colors I want.
I then simply pick out a few colors for my piece and make a "color sample" layer (this way I can easily pick out colors in a fast way by using the "eyedropper" tool (I)).
Normally I add a layer with neutral grey below the sketch layer (It's important that you either remove the white background leaving the black lines or change the layer's blending mode to something that leaves the white out). Then I paint rough highlights and shadows by using a soft/hard brush and just white and black (you can easily switch between theese with "X"). Again, if I need another color than black or white I choose a mid-color-grey with the eyedropper tool directly from the drawing.
Next I work out rough colors. Always start from dark->light (remember to use the eyedropper tool to select colors from your sample layer).
After you've added some rought colors you can start adding more detailed colors by either using the eyedropper tool directly from the drawing or your sample colors. (remember when adding highlights, their rarely 100% white)
Also, you might find a need to turn up the opacity of flow(opacity/hardness as you near the finishing steps of your drawing (but try not to over-do this).
A finishing move that I like to do before saving is to add a layer with grain in it. This makes the drawing look less 'computer-painted".
Add a new layer filled with the RBG-values: R:128, G:128, B:128. Then Add “Noise” to the filter, 400% amount, and set the layer to “Overlay” blending mode. Now run the “Brush strokes-Spatter” filter a couple of times.
Do a normal blur and set the layer “Opacity” to 5-10%.
hmm.. Sorry if some of those lines makes no sense, but I don't have time to check my grammar, have to go
Just ask if you need more help.
Oh! And after scanning and opening up in photoshop, you might want to change mode to RGB (image>mode>RGB I think .. :S)