I'd like to use Inkscape to blend some semitransparent shapes in RGB, so that R+B+G = White. Like this:



Just I'd like to do it without putting a white shape in the middle like is done in that SVG file. Help would be appreciated.

Draw them fully opaque, and set the blend mode of the layer to "screen".

Actually succeeding at this required me to set the layer blend mode, and then shift the objects back and between layers to get it the blend mode to affect them. And I found that overlaying semitransparent objects in 0.47pre is buggy.

Create 3 objects, R, G and B, and put them on separate layers. Change the Blend mode of the top 2 layers to Screen. Layer options can be found under Layer menu, to change the Blend mode you need to open Layers... dock

Thanks, that works great and I learned something new about Inkscape.