When using a mouse, that is definitely not the most effective way to go about it. You've already got a nice drawing with values, use that instead of forcing you to paint unnaturally with a mouse. Put the layer with your sketch on a separate layer and make sure it is the "highest" layer, on top of every other. Set this layer to "multiply" (other modes may do the trick too, but multiply is easy.)
Then on the layers underneath that layer, block in the colors. You'll see that the tone of the greyscale image is preserved. You can now start tweaking the shadows, and when that is to satisfaction, create a new layer on top of your original image, and add highlights on that layer.
At least, when "painting" with a mouse, and using the resources you have, that's how i'd do it.
But if you're serious about digital painting, get a tablet. Get it second hand, get a crappy 50 dollar/euro/whatever one. As long as its got a pen and pressure sensitivity, it's already a million times better then any mouse could ever be. Accuracy is less important then pen pressure.
As far as feedback on the actual picture goes, don't be afraid to use some contrast, especially darker shadows.