best way to colour with ps7, is to first get a good quality scan of the freehand work, then once youve done that you need to tidy it, (get rid of all the scribbles etc), I then normally if poss blend it to a white backdrop, once thats done its just a case of building up your airbrush layers till you got some colouring goin, tho in saying that a base coat of greys/whites can help in colour deffinition, aswell as useing the crop or freehand tool to fill larger areas