I think we're talking about different things here. I'm talking about finding lots of purples in the exchange, which would have been bound and unavailable to the exchange if the people who'd picked them up had used them. But a pretty narrow range of items (espeically for space) is going to be useful to any given character, and a lot of items are just crap.
Items would have a colored box (5-pixel-wide line with solid color) around their icon that would relate to how many unbound versions of that item exist on the server, whether in banks, exchange, or player inventories. This line's color would represent the over-all "supply" of the item server-wide. So if you had the only unbound one in existance, then it would appear as a shimmering gold line, let us say. And if there were 100 or less, purple. 1000 or less, blue. 10,000 or less, green. More than 10,000, white.
Then basically if you bind the item, then it would lose its rarity line, because once something is bound, its rarity becomes totally irrelevant because it can no longer be traded.
Meanwhile, the colored "sheen" that items' icons currently posess would be renamed "quality" instead of "rarity." This sheen is really just a representation of the item's quality; a shield with [Dis] gets green, while if it has [Pla] it still gets green even if there are hundreds of times more [Pla] than [Dis]. Same quality, different level of rarity by far; the system makes them the same color (green) because they are the same quality. So call it "quality," not rarity.
The fact that higher quality items drop less frequently in random drops does not mean that the system colors them the way it does because they drop less frequently. It just means that it bases the likelihood of drops on their quality. So in random drops then there certainly is a correlation between quality and rarity, but, the color scheme is still derived from quality, with the rarity correlation simply being a byproduct of drop ratios. Whereas with mission rewards, they are usually high quality (blue) but are extremely common and worthless because everyone gets the same rewards.