Use the vertex colouring tool in polyworks, but set the colour to white (to lighten) or black (to darken), give it a low opacity (~10-20%), then you can click on vertexes to lighten or darken without having to choose the colour each time.
Can also do the same thing to tint the colours slightly.
Even if you aren't trying to create the effect of a light source somewhere, varying the colours even slightly makes the map look much nicer.