I remember seeing and interview with Stan Lee where he was asked that question. From what he said it was a conscious decision to have the Hulk wear purple pants. Purple is supposedly the color that depicts rage.
Sounded like a good answer to me.
Sometimes things are much more enjoyable when you dare to suspend disbelief.
ahh i htink its just because purple and green mix the best, as for stan lee's answers on his comic books he always give some ancient philosophical answer or some adaptation of himself in his answers , which i personally thing is a load of crap he just says what people wants to hear, he doesnt know where he got this stuff, he even says how he got the fantastic four and spiderman were just trial and error and weeks of thinking what would be a good superhero