I've noticed that different pieces of armor for my characters, when equipped, turn the slot that the armor is in to a purple or yellow shade. What's that mean?
Yellow means that you won't get any benefit at all from wearing the item because it has a tag (race, class, prestige) which doesn't match your character. Prestige tag means you need All Access to wear the item, race/class should be self-explanatory. I forget exactly what turns stuff purple. I think it may be from having a prestige augmentation in non-prestige gear without being All Access -- the aug wouldn't give stats but the gear itself still would (I think).
The coloring can also be due to level restrictions, in addition to race/class/prestige. And there may also be items with deity restrictions. Tat
An item with a augment you cannot use will be purple (item is usuable but augment might be prestige or have a level restriction). In that case, you get the benefit of the item, but not the augment.
Last time this happened to me, I did not get the benefit of the item. I had an aug which I didn't notice was above my level. Instead of gaining 200 HPs (from the aug), I lost 2000 HPs (the value on the armor piece). Tat