Is it OK to use gray PVC electrical conduit for low voltage wiring?
Your method is legit and there’s no need for additional markings. All our phone and Ethernet are in EMT   I paint it blue to distinguish.
You can spray paint it if you really like overspray, but it seems to me like a recipe for a hatchet job unless you use a LOT of paint. Hatchet jobs are a code violation, NEC 110.12.
I would brush/roll it after dragging the PVC through some green Scotchbrite to mar the surface so the paint has something to grab.