Yup, Appaloosa is definitely a breed and not a colour. You shouldn't exactly call a horse Appaloosa coloured. There are different permitted colour patterns in the Appaloosa breed, many of which are also seen in other breeds. For example a leopard Knabstrupper. I can just imagine the owner of said Knabstrupper being a tad offended if you call it an Appaloosa-coloured horse. :P
His facial expressions make me think of Appaloosa. Also the shape of his head. And then the mottling around his muzzle and the bit of white sclera visible. I am thinking the white on his bum could be a lace blanket which would have been more pronounced if he wasn't skewbald. But all of this could be coincidental too, I suppose. ;) Maybe have a look at the temperaments of Appaloosas and see if some of it sounds familiar.
I think this is quite a well-written article about them: True Appaloosas - A word about Appaloosas, Training Horses, About True Appaloosas, Smart Independent Intelligent Appaloosas Horses.
As to other possible breeds, I have no idea, sorry!