Hmmm dunno what i should say.
First of all create all the layers for where u want to colour e.g hair, scarf
so when u mess up u dont have to start all over again.
I set all my layers onto multiply so that when u paint the colour wont go over the black.
Then select the layer u want to paint and then paint.
After u painted everything make a extra layer for each layer for shadows/ shading and set them to multiply aswell.
All u have to do is to paint over the parts where u think should be darker with the same colour as the orginal layer. If you dont think the shadows are dark enough just duplicate the layer it should turn darker.
After shading use the dodge tool (the tool that looks like a black lolipop) and go over the places where you think there should be light.
It'll look a bit rubbish in the beginning but once u get going it starts looking extremely S.E.X.Y xD
Also, if u want to get rid of the grainy parts after colouring select the background layer and then use the rubber tool and start rubbiing them out.
You can also use the burn tool to create some shadows but dont go too crazy on it or something bad will happen

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Oh crap i forgot, make sure the mode is on RGB and duplicate the background layer and delete the orginal one.