Those are great maps! I love the iso keep. You should start a thread on them over here and do the colouring.

As a first step I place a parchment texture behind the images. That gives one overall colour to the piece and some texture. Then I use a soft light or overlay layer above and some low opacity brushes to lay in the colours. Finally andother overlay layer and a dark blue to place the shadows then a very light yellow to place the highlights - with a more saturated yellow for the areas directly lit by fire (or lava). You can have all these layers at low opacity and it will look almost watercolour, or build up much more saturated colours like I have for these.