Turn off the brush settings, then paint a flat area of colour for the face. Press Cmd/Ctrl and click over the layer thumbnail, then select Layer > Layer Mask > Reveal Selection. Set this mask on a layer group so you can’t paint over the edges. Use your textured brush to add subtle light and shadow.
When painting a face, keep the brush’s opacity low. Rather than blocking colour in, you’re subtly building the details up, using a variety of tones and strokes. Always paint with the largest brush you can, gradually decreasing as you need more accuracy.
Paint the hair in a flat colour, and as with the face, select the layer’s contents and make a layer mask. Build a variety of coloured and contrasting streaks into the hair over a few layers. Use a large soft brush on the layer mask itself so that you can give some translucency and softness to the edges of the hair.