From what i've read it isn't a color, just a pattern - but no registries acknowledge them except the buckskin registry and they call them duns, which isn't always the case. It will be really interesting to find out more about this as they do more research!!!
I guess they had their own registry, but looks like it was privately owned and is now out of service.
Here is the most informative website I've found as of now. Not that well put together, but not a whole lot is available on these horses. If anyone else finds any info on brindles I would be great if you would share it :)