Roan in American Warmbloods
I noticed that roan was not an accepted marking for American Warmbloods which is fairly odd given that American Warmbloods are more of a "type" than a breed. Any color is acceptable, especially since their lines are open to several breeds that do carry it.
As per the accepted outcrosses: "Any cross or purebred of KNOWN parentage. No purebred drafts. Ideally should be of sporthorse type. Common outcrosses are European Warmbloods, TBs and Warmblood-type American breeds such as Morgans and Saddlebreds. Horses submitted for registration should have a lineage consisting of at least 25% of one of these common outcrosses (Warmblood, TB or one of the Warmblood type American breeds such as Morgan or ASB)."
I've only been able to find one roan American WB but he is registered with the American Warmblood Society.
BLUE CHEVAL - American Warmblood
FLASHY BLUE ROAN GELDING REGISTED AWS
It for sure would be a rare color but it is accepted in real life.
Our current American Warmblood, from my understanding, was modeled after the AWR registry and not the AWS/AWSSR which is more similar to International Sport Horse. AWS is a much looser registry as far as bloodlines, breeds, and registration (see here) campared to AWR standards (see here, page 4). Many other warmblood registries do not accept AWS horses as outcrosses, only AWR registered. In fact, if you look at the WBFSH members, it only lists AWR registry and not the AWS one.
So, to me, a roan AWR-registered horse would be needed as proof and not an AWS one or a better distinction between the two should be made (though International Sport Horse already exists--it would be similar to calling the ISR-OLD registry [the registry that was the main model for our definition of International Sport Horse] the same as the OLDBG one just because they both use 'Oldenburg' in the name).
That makes sense, I didn't know which registry we were basing off of. It does make sense that we are using the stricter registry, thanks for pointing that out Aylet!