How do I make an area in a video darker?
If I may, I would recommend using Davinci Resolve for this. It's free and does what you want way better than premiere. Since your question was raised for premiere though, let me propose two different solutions for this.

Use a Luma-Matte for an adjustment layer. To to this:


Duplicate your footage
Apply curves on the footage to maximise the contrast. (the wall should be completely white, your talent should be completely black.
Apply a curves effect on the adjustment layer and place it below your duplicated, contrasted footage and above your original footage-layer.
For the adjustment-layer, under &quot;track mattes&quot; select the luma matte from your contrasted footage-layer.
Now dial down the curve on the adjustment layer and see that it only affects the wall.


Use a Roto. If the Luma-Matte isn't working well because you can't really separate the talent from the background, this would be your last resort.


Draw a mask around your talent's face on the footage-layer and do that for every 2nd to 3rd frame, so that the mask moves with the face.
Apply curves to your footage-layer.
Press &quot;e&quot; to bring up the effects on your footage-layer and open up the curves.
Under &quot;Compositing Options&quot; click the small &quot;+&quot; and select the mask you just drew.
Make sure the mask is set to subtractive if you masked out the face, or additive if you masked out the portion of the wall.
Now dial down the curve in the Effect Controls and see that the effect only applies inside the mask.
You might want to feather the mask to not have a hard edge that might be distracting.

I hope this helped, have a good day! :)
Edit: You might want to use a mix of the two methods: The Luma-Matte for the hair, which is hard to rotoscope, and using masks for the simpler areas of the head.