Make sure you keep the refine edge brush small - it is for edges and if you make it too large you can end up taking pieces out of the wanted area.
Note , if you do get contamination, I would not use Decontaminate colours. It is destructive and does not work well.
Instead , add a new layer abobe the masked layer from Select and Mask. Set the blending mode of that new layer to "color" then clip that layer to the masked output layer (Alt click on the border between the two in the layers panel). Take a soft brush and pick up the hair colur (Alt click ) and brush on that new layer. It will colour the edges without going outside the mask.