GURPS Alpha Centauri is based on Sid Meier's Alpha Centauri, isn't it? The predominant color of the vegetation in the game is red (since it's fungus and not necessarily reliant on photosynthesis, in theory) - though that depends on how much they've riled up the ecobalance. There's plenty of green, though - even without terraforming beyond possible seed probes the ground is mostly dirt (from previous fungus cycles?) with a smattering of green (fungus activity rises when Alpha Centauri B comes into perihelion - perhaps because it handles the different light better? Or is it Proxima Centauri that enters perihelion...for which all bets are off)