~ You start with a clean image in black and white.
~ Then you "Promote Background Layer".~ Set this layer to "Multiply".
~ Ad a new raster layer and put it on the bottom.
~ You now have 2 options, you can use the paint brush to colour in places free hand or select an aria from the drawing then switch to the colour layer and increse the selection by 1 or 2 then fill in the selected parts. This last method allows you to go faster but the first one allows you to blend colours and make it more fancy.
~ The bottom one I did in 3 layers then used Eyecandy Glass on the 2 colour layers then merged them - then selected the white part and inverted the selection and did glass again over top with no map on the glass this time.