Additive Emission in Cycles Question is how to create additive shader. 
If you need particles which just "add" light to a scene. Not render a surface but create emission. 

If color changes from yellow to red for example, the red area of model does over-draw the yellow area (object behind) in case the emission does not understand to just add / increase color factor.



My project: Download Blend File (sorry, stackexchange responded with upload error)

How to math "add" color in cycles? 



Like Halo in Blender Render.
The way to do this is to add a transparent shader to the emission shader.  This means that the surface will emit light and allow light to pass through it, so further emission materials behind it will add to the brightness.



Adding some randomized color produces this result.  Note how the overlapping portions do indeed add up brightness as desired.