I wouldn't need this if I was merging two colors with 255 opacity but in this case I am pretty sure one of the colors had an opacity of half or 100 or something. If a(background) + b (light effect on top, with lower opacity) = c, I have a and c, and I just need to find b at different opacity levels (they don't need to be too precise, can be 3 intervals like 70, 123, 180 or something)