This is the way I was taught / understand the definition of these breeds(yes I know it's old school, but that's why I'm asking).
Apps: had to be a "stock" type build to be registered, otherwise it was considered a cross. There was some disagreements as to whether or not non-spotted Apps could be registered.
Paints: Stock type horse with a certain % of white above the knee. Breeding stock paints were not valued as highly since they only carried the gene for color, but didn't have color themselves. The issues came into play when there were "old type" Morgans & QH's that had excess white and they were then registered as paints.
Pinto: Basically the same as Paint, however the registry allowed ANY type of horse with the paint markings. There was no breeding stock classification that I can recall. Also, if you have a horse with paint coloring and unknown parentage, you can also register it as a pinto.
Buckskin/Dun/Grulla: Those registries were only for stock type horses. Buckskin was a black base w/ "yellow" coat, black points, and NO dorsal. Other "dun factor" was acceptable. Dun was basically a red base with a dorsal, or the yellow w/ a dorsal. Grulla was a primarily black w/ silver hairs and a dorsal.
Palomino / Cremello: stock type with a "blonde" base.
I've heard people say that Paints & Apps are a seperate breed while buckskin/palomino is not. Here's where I'm confused. I have rarely, if ever, seen a TB that had Paint, App, Buckskin coloring. Not to say there isn't, I just don't recall seeing any.
You can have a horse that has NO paint genes anywhere, yet come out with what was originally called high white, and suddenly it's said that it has whatever paint gene. So you may or may not get paint if you breed for it. Same as with buckskin, it's a crap shoot.
On the other hand how can you say that paint/buckskin/palomino is anything but a color breed especially considering that the coat patterns show up in many different and diverse breeds.
I used to have a McCoy arab mare who had a large belly spot. As far as I know, that line did not carry paint genes. However, when I sold her as a broodmare to pinto-arab breeder and they had her tested, she had a paint gene. The genetics people said that a paint gene can be a mutation, just like buckskin or palomino making horses with those coat patterns/colors "Color Breeds". So what makes certain colors/patterns a color and not a specific breed while others are considered a seperate breed, not just a color?
Clear as mud, right?