Question: So far in Dead Island I've noticed white, green, blue, and purple items. These seem to indicate different rarities, similar to the classification system used in Borderlands, but I don't know what these colors actually mean. Is green more rare than blue? What are the different rarity colors?

Answer: 
So far, here are all the colors I've found ordered in quality (based on overall stats when compared to one of lesser color and also how often I see weapons of that color):

white->green->blue->purple->orange

The in game text during loading says that some of the quest rewards gives you unique weapons, and (assuming Gabriel's Sledgehammer is a unique), it is still an orange colored weapon. So I'm going to assume that even unique weapons are orange. I've also found a Master Chef, which was also orange.

[This link](http://www.g4tv.com/thefeed/blog/post/716093/dead-island-weaponsmods-location-guide/) seems to back up my answer.
As Ashley Nunn points out in an [answer to another question](https://gaming.stackexchange.com/a/46137/8366), the Dead Island wiki [explains each color in detail](http://deadisland.wikia.com/wiki/Item_Rarity):

> 
> There are a total of FIVE different levels of rarity for an item in Dead Island. These are:
> 
> 
> * White (Common) - Absolutely everywhere, most items and weapons are this color.
> * Green (Uncommon) - Frequently found in chests and given as a reward from quests. Uncommon items are occasionaly [sic] dropped by zombies on death.
> * Blue (Rare) - Often given as a reward from quests but can be found on zombies after death and in chests although the chance is low.
> * Violet (Unique) - Occasionaly [sic] given as a reward from quests but can be found on zombies after death and in chests but the chance of this is very low.
> * Orange (Legendary) - Rarely given as a rewards from quests and is both rarely dropped by zombies on death and found in chests. Finding one of these is truely [sic] a gem, congratulations if you have one.
> 
> 
>