Red: Obviously prefers hot places. When it rains, steam is said to spout from the tip of its tail.Yellow: The flame at the tip of its tail makes a sound as it burns. You can only hear it in quiet places.Gold: The flame on its tail shows the strength of its life force. If it is weak, the flame also burns weakly.
Prof. Lotus' Notes: Charmander being the most popular of the 3 starters is attributed to more than just it's evolutions, because in 1999, most players didn't really know, nor care about the evolutions when starting. Charmander is actually another example of good video game design, where a psychological technique known as the Center Stage Effect, the selection of products based on the one in the middle - deeply routed into our psyche. The reason for this is Game Freak wanted players to not just rely on one super powerful Pokemon, but to actually go out there and catch more Pokemon that would cover their needs, instilling a behaviour early on.