Create a new layer. Call this 'Dark Tones' and set its layer mode to Multiply. Selecting the mid-tone flesh colour and using a soft-edge brush tool, paint in the shadow tones. Now lower the layer opacity - here I used a value of 70%.

(top left)
Dark Tones layer seen on its own.
(top right)
Dark Tones and Base Tone layers viewed together.
(left)
All layers combined.

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