Combining RGB layers

[GnR]

I also tried what appears to be the same approach with black background and lighten only, but the total output was more than 100% lightness (original yellows were washed out). I then added a greyscale for lightness only, and reduced each layer to 25% - this produced the right amount of lightness overall, but average hue was incorrect.

I'll take a look at the script and come back tomorrow, thank you.