Red: When the bulb on its back grows large, it appears to lose the ability to stand on its hind legs.Yellow: The bulb on its back grows by drawing energy. It gives off an aroma when it is ready to bloom.Gold: Exposure to sun light adds to its strength. Sunlight also makes the bud on its back grow larger.
Prof. Lotus' Notes: The Bulbasaur line is an example of good video game design. Without the Red or Blue games implicitily telling you, Bulbasaur was the easiest option, Squirtle Medium difficulty and Charmander being most diffult because of the first two Gym Leaders being Brock (Rock Type) and Misty (Water).
Ivysaur is most likely a portmantaeu of Ivy and Dinosaur. It's Japanese name being フシギソウ (Fushigisou), a play on words of 不思議 (fushigi) 草 (sou) - strange grass. It's called Herbizarre in France.