Red would really clash with the trees. Black could be quite beautiful, though. Also, snow would melt faster because it would absorb more light.

Wouldn't clash with the trees that shed their leaves, though. And red-and-green pine trees is very Christmasy. :)

Fast-melting black snow might lead to sea level rises and droughts.

They both amount to the same result

Somehow I suspect that in a world of perpetual halloween the snow might be red.