Doc:2.6/Manual/Render/Cycles/Nodes/AdditiveColor

Cycles Additive Materials

If you used game engines or other renderers. You know about Additive Blending for materials.

Also, the same material is in 3dMax. http://docs.garagegames.com/artist/official/3D%20Studio%20Max/AddiBlend.jpg

And here it is how it can be done in Cycles:

Simple Setup

Without Emission

With Emission

Another good additive Material approach