COLOR ASSOCIATIONS (Color Symbolism)

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

http://en.wikipedia.org/wiki/Halloween

"Black and orange are the traditional colors of Hallowe'en. In modern Hallowe'en images and products, purple, green, and red are also prominent.

The use of these colors is largely a result of advertising for the holiday that dates back for over a century. They tend to be associated with various parts of Hallowe'en's imagery."

I have not yet found any supporting evidence on how these colors came to symbolize the above. We must note, however, that red, orange, brown, and yellow have typically been associated with Autumn in many areas because the leaves on trees change to those colors.

The meaning of the association of using orange and black with Halloween goes back to the Druids.
"The traditional colors of Halloween reflect its Celtic origin. Orange is the color of the autumn harvest, black the symbol of death."
BTW...This answer was obtained on the internet. If you'd like to read more, go to this website:
http://www.watchman.org/profile/halwnpro.htm

According to the pagan perspective, black represents neutrality (that is to say, neutralizing dark or energy) and is associated with Hecate, goddess of the dark moon and of witchcraft. Orange is representative of strength and endurance, and can be relevant to the harvest when observed from the view of withstanding the winter and enjoying the fruits of one's labor.

There are other opinions on the subject, but anything involving evil about those two colors is false; the veil IS thinnest on Halloween and anything attracting evil is ill-advised.

Halloween, or the eve of All Hallows day, 1 Nov, also known as All Saints' Day (NOT to be mixed up with Halloween like some sites do) was the night of the year when dark forces were at their strongest, just before the day when the influences of good held sway. This is where (and not, as some think, from shipping) the phrase "The day is darkest before the dawn" comes from.

Because of the evil that lurked on Halloween anybody who had business abroad during darkness would put on a disguise they would 'guise themselves - so that the evil forces would be fooled into thinking that they were also evil. They often carried with them a lantern made from a swede turnip, hollowed out and with features cut to make it look like a skull. The flames from the candles would bathe everything in an orange glow.

Now children like to be scared a little, so they would also dress up and go 'guising to the nearest houses trying to scare them, and the owners would offer a small token to appease these obvious demons. That is where the original idea of the American 'trick and treat' came from, but it has since corrupted into trick OR treat. In Britain the tradition was for the children to be invited into the house, where they could participate in ducking (for apples) et al. They would be expected to perform a small rhyme or song before getting their treat to show they were indeed human and not demon.

The black, associated with night/evil/death is obvious, but the orange never came from pumpkins or the colour of autumn, but from the light of the candles burning in the turnip lanterns, which are only called Jack o'lenterns by Americans. Halloween dates back several thousand years and has only become commercialised in the last 50 or so years, slowly at first but noe at the point where it is another Christmas for the manufacturing companies. And so the message has been obliterated and children guise without knowing why or even caring why.