There are two issues with paint on tanks.
(1) Paint hides stuff. Realize that any time you paint you potentially provide an area for undetected rust or corrosion. Not such a big deal with AL tanks, but some serious rust can grow under paint on steel. For that reason use a paint with a rust inhibitor in it - and clean the tanks clean, clean, clean and make sure they're absolutely completely totally dry before painting - and don't paint on a humid day.
(2) Temperature - many paint processes - especially commercial - involve baking on the paint. This is an ABSOLUTE NO-NO with scuba tanks. It weakens the metal and can and HAS resulted in tanks exploding and killing people during fills. For this reason, fill stations may be reluctant to fill any tank that isn't factory painted with a color they can recognize as a regular factory color.
Painting at room temperature with epoxy or spray paint doesn't warrant a hydro, unless your prep has involved sanding - sanding the tank warrants a hydro. You may have to get a hydro after painting anyway before the fill station will fill it - the last guy I know of who brought in a baked tank to be filled swore it wasn't, and the tank blew - and most folks in the tank filling business know it. So if they insist on a hydro first even though your paint job is obviously not baked, go on and bite the bullet - after all, it's their tail on the line when they're filling.
Rick