This one is indeed odd! If you look closely, the marks seem to be just like dapples, only reversed. Most dapples are lighter than the base coat. Some roan horses get dark dapples, and this might be what is going on, but then she is a VERY unusual roan because of the distribution of the roan areas. Donkey roans do this routinely, but this ain't no donkey!
Have to remember this is a place where horses are allowed to run wild. Her mother is a gray, very light looks white almost. the only possibilities to be her father could have been a very light blue roan colt, or a very light dun roan colt, the dun paint (Jefito), or her half brother, a light gray with a few fading appy spots on his butt. These were all young boys out here at the time. I know the dun paint, and the light dun roan, were both pretty determined at a young age and fathered a few foals, the other two were not so, and don't know of either one of them siring any foals, though I could be wrong.
Could this be a new color mutation? The spanish Colonial Horse since its return to the birthplace of the horse has produced several original colors not found in Old World horses.
One could almost see this pattern as protection against airborne predators. Which then lends itself to the suggestion that it might be a throwback to an even older bloodline....like prehysterical? ;)
I second that motion...if I were rich and famous...well if I win the lottery..LOL...I would love to know if UC at Davis could do a DNA test on her and find out if its a new color or a mode of an existing color.