If you believe a post on Reddit, the Legendary items will change their colour to purple:
At first glance, the first two levels of rarity are the same, except with a better color. The 'sky blue' color in GW was too confusing on the screen. Some monitors displayed it like a sea foam green and others as a true blue.
The next level is backwards form the current MMO trend (WoW, WHO, etc.) and will cause some confusion, but will probably pass.
The Rare level actually makes perfect sense and matches GW. This will most likely be top level items for the level.
The Exotic level looks like a replacement for the green level in GW. I'm not sure why they replaced it but it does match WoW's item level.
The Legendary is the real puzzler as RDarken mentions that red=bad in most cases. I agree with this assessment.
And why remove purple? It is a solid color choice to use in the scheme (and also used by other games). I'm all for setting your own trend, but one of the reasons that WoW has remained on top (and WHO did so well), is that it kept a trend going and didn't deviate from what the competition and instead absorbed their ideas as a new feature.
I am not a big fan of the different colour coding of GW2, as it kind of collides with its own mechanic at crafting, where "red" means "unavailable", orange means normal XP, green less XP and grey means no XP.
I guess crafting, items, and mobs even should adopt a similar color scheme. Having different color schemes within a game is a bad idea. Thanks Gorani for noticing this.