Steps (just an example; there are other methods)
1 Duplicate layer → B.
2 G’MIC (e.g., try my Contrast FFT) and output to new layer → C.
3 Grey scale (e.g., Desaturate) layer C.
4 Make mask for layer B using layer C.
5 Select layer B and start painting over grey hairs.
Before
After (a tiny brush stroke to demonstrate)
