L.W.
Opposing what another mom said, I think it is the Halloween themes that he was exposed to that makes him think darker people are monsters, not the tv time. I don't know how much tv your kids watch but that is your buisness, not for others to judge. As for the color thing, show him the difference between mosters and colored people. say look, here are some monsters (example where the wild things are book or something like that) and show him another image and say these are people, just like you and me, they have normal eyes, a mouth like you, they have two arms, two legs, a nose...just point out the similarities between himself and them, and than point out the differences between the monsters and them. And of course tell him about how all people have different skin colors and that is great.
My son yelled "hey mom that lady is black!" in the store and I said yes, and black is one of your favorite colors right? and than he ran up to her and told her "look at me I am brown!" (we are native americans) and she was really understanding with him. Children are just like that, they are naturally curious, but they are also very excepting, and most adults know this and will not be offended.