Orange, black, and yellow (in that order) comprise the primary Halloween palette (in the U.S., anyway). Purple/green may be less obvious to people who don't think about this sort of thing, but it's ubiquitous these days, just walk down the Halloween decoration or card aisle at any Walmart. I remember these colors being used as early as the mid-70s, with green perhaps most common as witches' skin and purple for night sky. I'm wondering how far back these design conventions go.
Green has been a Halloween color in the US at least since the 1950s, quite probably further back. Green is the color of pumpkin stems, and since the 30s at least the "color of fear" (as per Frankenstein adverts) and of course the witch in Wizard of Oz. Once you have orange and green, adding purple is more of a design choice than anything else. Orange, Green, and Purple form a triad, which means they look pleasing and balanced together while still retaining a vibrant contrast: http://www.tigercolor.com/Images/Triad.gif
I don't see yellow as a primary color for Halloween. Usually orange and black. I always thought the purple (like the purple lights) was because it was the closest color to black and you can't have black colored bulbs.
