Notice that Red and Green gives brown (in this case 20/40/60/80% green on red).This strip is a reply to Variations on a 4 Colour Pallette, Purple and Green using Opacity and Inversion, SG for DUMMIES Contest!
In an RGB colorsystem (additive system, with light instead of pigment) Red and Green would give Yellow.
Edit: The reason why this not happens is that SG does not use a regular 8-bit RGB colorsystem.
Only the Red, blacK and White are pure.
Green and Blue are not pure colors in SG.