I have a green background picture with some shadows. I would like to use it as a mask so I can place it on background with any color (e.g. red or pink) and get the same style of shadows.

I tried converting it to grayscale and changing opacity to 20%, but this resulted in a lot darker image.

I am using this on a webpage where admin can set template color. Then I am using something like this in CSS:

background: #557d0d url("img/background.png") center center;

Using Gimp 2.8.6 on Ubuntu.