colour to alpha eroding picture's opacity

[Ofnuts]

To avoid your subject becoming semi-transparent, make a (counter-)selection to protect it:

1. Fuzzy select the background
   
2. Select ➤ Grow by a couple of pixels so that the selection include the edge pixels
   
3. Apply Color-To-Alpha
   

See Background removal or replacement on text, logos and other computer graphics

My ofn-erase-background script automates some of this.