Just wanted to agree with Kelti here. At one point I dyed my armor dark blueish... I have since changed my items out several times in most slots, and I can't get item colors back. I simply dyed everything white, which for leather items that I wear as a monk, means everything is the brown leather look. Since dying it white I have switched my BP at least to something that should be dk blue in color, but it only shows as normal leather.
So yes, once you dye, that item slot will always be dyed, the only thing you can do is change the dye. Even if you match the item's color in the preview screen, if you ever upgrade, the new item will be dyed to the color you last switched to.
Fun eh? The cost of uniqueness, you'll never again see true armor colors.