Why Is the Green M&M the Horny One?

Mars Company has been producing M&M Chocolate Candies since 1941. Various rumors have since been attached to different colors of the candy: the green ones are an aphrodisiac; if the last candy out of a bag is red, make a wish and it will come true; if the last candy out of a bag is yellow, you should call in sick and stay home; orange M&Ms are good luck, but brown ones are bad luck. M&M/MARS notes that all these rumors were developed by consumers, not the company.

The rumor that these green candies are an aphrodisiac apparently started or first gained prominence in the 1970s, when students reportedly picked the green ones out of packages to feed to the objects of their desires.

Very key article on colour legends. Here's another:

“There has been a great diversity of opinion on the subject, but the generally accepted rule is pink for the boy and blue for the girl. The reason is that pink being a more decided and stronger color is more suitable for the boy, while blue, which is more delicate and dainty, is prettier for the girl.” [Ladies Home Journal, June, 1918]