You could try tightening up the contrast on the mask. ie not have such a gradual transition between black and white.
Yes, thank you for your reply! That helps a lot in some situations when a hard edge is ok. But when it comes to hair and some clothes/fabric it dosent look realistic with a to hard edge. And the crazy thing here is: if i use a grey, green og maybe blue background color I do not face this problem with the backround totally changing into another color when blending into the soft edge of the layer above the background. I have done this with composites for many years without this strange colour shift happening. This is the first time I experienced this issue, because the client want a purple background.
While it doesn't happen regularly, I have seen this issue over the years where certain combinations appear to blow out specific colours and/or the saturation.
You could try adding a Hue/Saturation adjustment layer and clipping it (Layer > Create Clipping Mask) to the portrait layer below it. Reduce the saturation within the Hue/Saturation layer and use the mask to paint away the saturation in the problem areas. A very manual method but it should do what you need.
good idea! Desaturate and even use the hue slider to match colour of hair/fabric maybe.. Thank you for a good tip! I will give it a try.
Good call on also using it to tweak the Hue. Best of luck and let us know how it goes.