Just to inform the discussion a bit more, I tested how a couple of other programs handle lighting and overlap.
The first one is Roll20, which as far as I could see doesn't have a colour control for lights? (It's been a year since I used Roll20 as a GM rather than a player so I may simply have missed it).
They do seem to have a bit of the "lens" of light that's too bright in the overlap, but is is soft-edged and rather subtle - I'd have to try it on a flat grey background rather than my standard map example to be 100% sure it's the lighting not variations in brightness on the example map.
The second one is Foundry.
I would say they are doing a slightly worse job of the saturated red + saturated green = saturated yellow, but it is avoiding colour clipping on the bottom left.
The desaturated lights, bottom middle, look to my eyes as though they are colour clipping right in the middle, but it's softer-edged and more graceful looking than the FG example in my opinion.
Where they definitely are doing a better job is what I would think of as the most common use-case - two overlapping lights of the same colour, in this case a moderately-saturated yellow that I'd use for candlelight. There's no hint of colour clipping in the overlap there. I'm not sure how they have achieved it, but there's no question that the result there is significantly closer to physical reality than the FG results using default FG settings for the candles.
I don't know if Foundry are doing this by better rendering model or some sneaky tweaking of default settings to avoid the issues, but I definitely think it's worth a second pass at the rendering model in FG to try to avoid the ugliest colour clipping problems.
Cheers, Hywel
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