From UOOutlands Wiki
As an addition to the taming system, tamers can now breed mounts in search of darker and rare hues. Every mount which spawns in the wild is assigned a gender, and the lightest hue of each range. Mount breeding takes place in your home associated with a special Mount Breeding feed trough, and requires a male and female mount of a similar species.
Crossing two different colored mounts can result in a 50% chance of any hue from either parents hue range, and a chance at a rare hued baby. The goal being: mounts only spawn in the lightest hues in the wild, so to get darker hues you need to breed them. To get a rare hue, you need persistence!