Simulating a "substractive" blend mode?

Is there any elegant (read: not too taxing on performance or data volume) way of making an object behave the exact opposite than with the "additive" blend mode?

For example, if the object is a pixel with the RGB values 0-50-0 and it's in front of a pixel with 100-100-100, I need the resulting pixel that's actually displayed on screen to be 100-50-100.

I've experimented with opacity settings and gamma values in the overlaying object, but they don't seem to work in any way that can give me the exact results I want.

OK, in the unlikely case anyone else wants to do this: There's a layer effect that is exactly that.

Wonder if there's a specific reason why it's not available as an ordinary blend mode, though...

The blend modes are something that will work even without webgl. So without webgl that's the limit what can be done. Once webgl is supported absolutely everywhere subtractive could just as well be a blend mode I'd imagine.