I always start out with a really quick line art sketch and work on perfecting that and getting the lines right. Then I proceed to add the colours, followed by 'wear and tear' affects. (faded areas...I typically don't do this unless I'm using specific brushes) Then I proceed with shadow and lighting affects. After all that's said and done I finish up with some color corrections.(Mostly darkening areas to emphasis what I want to be seen)
Also note: Sometimes I will draw out a specific portion and color it and add affects in one go, then proceed to another portion and do the same. It totally depends on how you like to draw.