Well I personally think it is a colour... I know in the US there are a few 'Palomino' registries and the UK has one now. To register your palomino with them they have to be a DNA tested palomino and be between a certain shade of palomino (so no Isabella or chocolate palominos). The horse can be any breed though. I think they are aiming to establish a 'Palomino' breed by selective breeding, after all that is how all breeds started out. Personally im not for breeding for colour.
As for your mare, she does look like a pure mare but without papers you can never be sure. I assume there was no information with her, if she was rescued?