It's widely studied and proven that people are extremely visual, which is why companies spend so much time and money on logos. The color purple is distinctive the the grape, since most other berries and fruits are some type of red. When candies were first produced, they were only colored with naturally occurring colors, such as fruit juices. If someone wanted to give candy a grape flavor and still make it visually appealing, they used the purple grape. As a result, it's become a universally accepted idea that purple means grape.
So it stands out and you know to avoid it at all costs.. ick.
Probably because they are using the color from the purple grapes as a reference, and so they can use green for lime. Also since grape has been purple for so long, if they changed it to green I bet most people wouldn't think it was grape and assume it was lime.