this is probably only useful to game design artists but that said; if (like me) trying to get tonal values right gives you hives, greyscale it first.
block out rough tone value changes in greyscale ( as draw the rough shape of where the the tone changes) and use your smudge tool to blend the tones together as necessary. This should give you the right tone values with just enough transparency that you can colour block on the layer behind and faff about with layering colours to add variation and light as necessary
example:
greyscale, no colour. greyscale with colour









