Way to use only part of the background for harmonization. Transparent images harmonization

[Vozf]

If we use 2 transparent images and try to harmonize them with each other we will have to fill the transparent regions with some color. If we use black color the harmonized image will become darker because of that. Is there a way to select only part of the background for harmonization(non-transparent part of the transparent image)? Like additional mask for instance. If not is there a preffered way to fill the transparent regions when harmonizing transparent images?