Duplicate colour layer..........Ctrl +J
Image. Adjustments. Black and white. OK.
Click adj layer icon at base of layers pallette and click B/W, enables you to go back, change anything or bin it!
Can bring back some colour of whole image as wish by reducing opacity of B/W layer, top R in Layers pallette.
Add layer mask to b/w layer......either click washing machine at base of layers pallette.... or..... Layer, layer mask, reveal all.
Black foreground, in colour picker.
Click B ...brush......size to suit. If need accurate painted back bits use hard brush ...Shift + ] .
Or, paint back some colour by reducing brush opacity in top bar.
Make sure painting on MASK by clicking on it, not on thumbnail.
If go too far, click X [changes foreground colour to white] and paint to restore B/W.