This is an array of item quality colors, used to color the item name any place it appears.
You can also refer to them using ITEM_QUALITY_COLORS[X]. The "description" of the item is given using the constant ITEM_QUALITYX_DESC, which will be localized. The NUM_ITEM_QUALITIES global holds the count of ITEM_QUALITY_COLORS.
The current list is:
0 - Poor
1 - Common
2 - Uncommon
3 - Rare
4 - Epic
5 - Legendary
6 - Artifact
7 - Heirloom
Patch 3.0.2 introduced quality 7 (Heirloom), but neither ITEM_QUALITY7_DESC nor ITEM_QUALITY_COLORS exist as of 3.0.3.
Update: As of 3.1.1 it seems ITEM_QUALITY7_DESC is defined as "Heirloom," however ITEM_QUALITY_COLORS still does not exist. Calling GetItemQualityColor(7) does return the correct colors.
Update: As of Patch 5.3.0 ITEM_QUALITY_COLORS and NUM_ITEM_QUALITIES both go to 7.