I'm definitely not a professional but somebody asked me about my technique a while ago and this is what I wrote for him -

I'm pretty bad at explaining my technique, and probably more than you were asking for, but I will try.

Repair and desaturate the picture first.

I use the pen tool to select the individual parts. After selecting the complete part, I save it as a path and give it a name (face, iris, hair etc.) so I have a nice little list of parts under the paths tab. When I want to color something, I select the path, right click on it and make it a selection. I usually set the feather radius to 2 pixels unless it's really tiny. If I want to do the face I will select the face then select eyes, lips etc. and subtract them from the face selection so they don't get colored until I'm actually doing the eyes or mouth. Once you have the part selected, go back to the layers tab and select the main layer that you want to colorize. There should only be only layer at this point. Open up the adjustment layer tab and add an adjustment layer. I mainly use the Color Balance or Black & White adjustments (check the tint button to add color). It's usually trial and error after that until I'm satisfied with the resulting color. You can modify the color further by going back to the layers tab and adjusting the opacity or the layer style (normal, soft light, hard light etc.) Just keep playing around with it until you find something you like. Since every part ends up on a different adjustment layer, you can go back and make change later fairly easily. Every part is done with a different adjustment layer (I usually name them if there are a lot) Make sure that you have the main layer clicked on for every new adjustment layer you add. One thing that took me a long time to figure out was that the paint brush acts as an eraser on the adjustment layers, the eraser just adds stuff. Make sure you save your work as a psd with all the layers still open. Once you are satisfied with how it looks, flatten the image by merging all the layers. I will usually make final small adjustments (color balance, contrast, etc.) on the flattened image. Make sure to save this under a different name so you don't lose all your earlier work if you need to change a color later. Hope this helps!