Isn't one of the paint registries a color registry? Where you can register based on color, not breeding?
I know they can be registered as pintos. Is that what you mean?
Pinto is a color registry. Paint is AQHA and Appendix QH (thoroughbred) lineage. Parents have to have been a registered Paint, or a AQHA registered horse.
We're just guessing that there isn't anything about Sarah Lee that would indicate she isn't a 'purebred'. But no one knows if she was ever registered, or registerable.
She could probably be registered Pinto, but she isn't the style popular among pinto people. Pintos tend to be Arab or National Show horse in type.