It will multiply, not add.
To correct:
Given (R,G,B), each color channel is a value from 0 to 1.
Pure yellow x pure blue will make black.
(1,1,0) x (0,0,1) = (0,0,0)
Pure red x pure green will make black.
(1,0,0) x (0,1,0) = (0,0,0)
Pure yellow x pure green will make green.
(1,1,0) x (0,1,0) = (0,1,0)
White x blue will make blue.
(1,1,1) x (0,0,1) = (0,0,1)
This is how standard 3D vertex-colored shaders work. To change how this works, you'd have to program a different shader yourself. Even Unity's sprites will work the same way.
