Diffrence blending mode
 

This is similar to substracive1 + substractive2.

Or as code:
red_value_out = abs(red_value_in1-red_value_in2)

:cry:
I posted a stupid reply and now I can't delete it

nice huh, the edit button? ;)

Nice idea Magic.X, maybe it'll come in avs 2.7 if they read it in time, can't be too hard to make.