When Shading complicated areas such as Black or dark colours (I'm come back to that later), or say a face, I give that layer four grouped layers. Working from bottom to top the first is the Complication layer.
For lesser areas 3, sometimes even just 2 are enough.
To start off make 4 new layers and group them (highlight layer and press CTRL+G) to the colour layer. Arrows should appear next to them. Now whatever you do to those layers it will only affect the layer it's grouped too, i.e. In this case on the main body green.
Sectionise your face like above. Or in the case of Black/Dark colours use Edit > fill (choosing foreground color this time) on your complications layer with a BRIGHTER colour.
THIS LAYER WILL BE DELETED LATER.
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