I think the safest description is “mostly”! As you say, basically light Sussex bantam, but here and there the odd red feather; at one point he achieved nearly perfect light Sussex plumage - apart from a single red tail feather. He arrived unannounced from a breeder across the road. They claim he escaped, but I suspect they tried to get rid of him because he wasn’t pure. Tellingly, when he first arrived he was short-tailed; on closer inspection it turned out the feathers looked as if they’d been cut, so I suspect they’d tried to pass him off as a hen. We took pity on (as we thought) her - then one day “she” started crowing, so Susie the Sussex became Soo the Accidental Roo.