How do I know if my subfloor is fire retardant or preservative-treated?
The concern is probably that adhesives won't bond to treated plywood, or maybe that color will leach out and stain the new flooring. You're right that you're seeing paint overspray on your subfloor. It looks like standard raw BC plywood.
Single-family residential subfloors are rarely treated, in my experience. This may not be true in some high population density communities with more rigid fire codes.
Fire-rated plywood is going to have that mentioned in a stamp on the bottom. There may not be a good way to determine whether it is from above. If you can find access to the underside from a basement ceiling you can probably see it.
Plywood treated for rot resistance is almost always green, and it's not readily available in tongue-and-groove configuration.