colored weapons

What is the difference between weapons that are white, blue, purple, green and gold?

Here was the original naming convention I believe in order or rarity

White = Common
Blue = Magical (dont let the name put you off, nothing magical about them, things from Item collectors are also blue.)
Purple = Rare
Green = Boss items
Gold = Uniques

People typically value gold items over all else though their stats can be replicated in any color, except white.

Item color was just a to add value to one item over another. You can find most weapon/items in any color, though some rarer ones only come in gold and very few of them only come in green.

You cannot change the item's name color from one to another. The only exception to this was taking a White item and putting a mod on it will render it Blue, or was I dreaming? I thought I remembered from all the way back when guildwars was only Prophecies that this worked. Someone let me know if this is true, I forget.

You can have the exact same item and someone will pay 10x more if its gold, compared to blue or purple.

I hope this answered your question

[wiki]Item rarity[/wiki]

Purples are uncommon, golds are rare, greens are unique.

Actually Green is unique and gold rare, GW wiki knows it all:
http://gw.gamewikis.org/wiki/Item_rarity

oh ok, my mistake

what these guys said

Thank you very much

In what way are greens unique?

They only drop from the one monster.