I believe what you are looking for is the alpha channel--the fourth color compon...

neild on April 26, 2011 | parent | context | favorite | on: Richard Feynman on education in Brazil

I believe what you are looking for is the alpha channel--the fourth color component which specifies how to blend two values. It doesn't make sense to add RGB(0,0,0) and RGB(0.5,0.5,0.5). It does make sense to add RGBA(0,0,0,0) and RGBA(0.5,0.5,0.5,0.5).