Hi, thanks for the quick reply!..
unfortunately, i've already tried doing this.. whilst it looks okay for light, somewhat muted colours, it's not exactly how light works (it's kind of like how it would work with thin layer of fog being lit up and washing over the existing colours... see how it brightens the dark "html" text?..when really black should absorb all light and remain black)
Here's kind of what i'm aiming for here:
even though this is a kind of garish and extreme example, (plus, again i don't think it's entirely accurate), it would still allow for much more vivid lighting (like deep red emergency lights)
The red blue n green are additive to each other, surrounded by black, on an overlay layer atop the 'before' image.
The more i'm thinking about it the less it seems possible... without the equivalent of an overlay blending mode D: ..unless, of course there's some cunning trickery