Black -- death, night, witches, black cats, bats, vampires
Orange -- pumpkins, jack o' lanterns, Autumn
Purple -- night, the supernatural, mysticism
Green -- goblins, monsters
Red -- blood, evil, devil
White -- ghosts, mummies

There doesn't seem to be any written history of these as colors of Halloween, they tend to be associated with various parts of Halloween's imagery. As the Celtics are given the most credit for beginning the holiday as the festival of the harvest, a color of autumn is used. Orange, being the most prevalent autumn tone, is derived from the pumpkin and leaves. Halloween was once (and still may be in some areas) a festival of the dead. Death is usually associated with darkness and absence of light signified by the color black.

Black cats, witches hats, cauldrons, and streamers are a few of the items that are reminders of the holiday. Most candies at this time of year are in black and orange wrappers. In our modern traditions a few colors have also worked their way in... Green, Red, white and Purple. Green skinned monsters lurking in shadows. White ghosts floating around. Red blood dripping from a vampires lips. Purple ribbons on the hats of witches.