The hues in UO are represented by a number (I think 8, but I may be off) of colours; like a gradient.
Then, based on the lightness of a specific pixel, the relevant colour is picked from the gradient thing.
For most colours, they run from dark to light of the same colour.
For Abyssal, it's not as simple as that. Not only is it not a single light-to-dark progression (giving it the almost velvety look when it's not doing the red), but it ends on the red, which means that the lighter parts of the object become red, whereas the rest dark grey/black.
The metallic dye tubs define this well, too, since you select two colours, and it does like a two-tone gradient.
It's like that, but black and orange.