Going by the Japanese versions:
Red is named after Pokemon Red, Green (Our Blue) is named after Pokemon Green. They're a pair. She is named after Pokemon Blue, Japan's "third version".
In the English versions:
Red is named after Pokemon Red, Blue (Japan's Green) is named after Pokemon Blue. They're a pair. We never got Green. Therefore, she is Green to us. Pokemon Yellow ended up as our "third version", but the game itself wasn't meant to be a "third version".
