help please Photoshop related

[Scooby_snax]

Help please.
I have a photo of my late father that my Mum wishes enlarged (I didn't take the pic)
The quality of the pic isnt brilliant and I wish to do some retouching
One of the finickity problems is illustrated here

there is a strange colour behind his head the rest being black, so i wish to darken the light bit but I don't wish to loose the whitness of his strands of hair (hope that makes sense)
So in effect I just want to replace the light colour with black
I have PhotoshopCS and Nikon NX to assist me
How do I go about it?
Thanks

[KenP]

Probably the quickest method is to use something like NIK Viveza and select the offending background colour and reduce the brightness.

[leeb]

could you not use the burn tool, set it to shadows and about 50%. it will only darken the dark areas

worth a go.