If you are still convinced, then yes, you are wrong about the colour of the horses. That doesn't matter in the long run, and is no reflection on you as a horse owner. The majority of horse owners are ignorant of the genetics of horse coat colours. That doesn't make them bad owners, it just shows that they don't really have an interest in genetics.
Kiger is still mustang, and still a domestic horse for all intents and purposes as far as colouration is concerned. The genetics are the same. The potential colours are the same. The display of the colours is the same. The horse you pictured has not got a copy of the dun gene, therefore, he cannot possibly be dun.
The two mares you have pictured are grey. Sure, the younger one could have been a red dun when she was born. Not disputing that at all. But she isn't one any more - she is a grey. Underneath she still has the dun gene, but the grey gene means she will eventually be completely white.
Question, what is a line-back dun? Simply any dun that has a line on it's back? How about a steel dust? What do they look like?