How to do this blending effect ?

[Espermaschine]

At first i thought this is simply a matter of finding the right Layermode, but nothing seems to result in white when blue and red are being overlayed.
Am i missing something obvious ?

   

[Ofnuts]

You can get white unless you have green and the green has to come from somewhere. In the image below, the "=Blue is in Addition mode, but Red is really (255,70,0) and Blue is really (0,185,255).

   

In your image the Red and Blue both have Green components that add up to 200+ (and the white isn't exactly white anyway).

[Espermaschine]

Wow !

You solved it in 5 minutes while i was banging my head against the wall for an hour

So this is all about the math ?! When you add up the RGB values of the blue and red circles, the sum of R, G and B is always 255 (=white) !