There are water based paints that can be used with an airbrush indoors, but that would only be suitable for low power rockets. Otherwise, you need to find an large parking area out of sight where if needed you can set up a card table that fits in your SUV or car trunk. As long as you don't act suspicious, you should be ok. You need fast drying paints and ideally the temps should be in the 80's with reasonable humidity. The biggest issue is you need a way to transport your rockets after you've painted them and the paint is still soft. I used a PVC stand that had a dowel for the motor mount. Really you could do it in almost any big parking lot away from anyone else. (But obviously not anywhere where security is an issue, like a mall or hospital.) Not far from where I currently live is a big park where I'm pretty sure I could find a place to paint without attracting much attention.
You really aren't doing anything wrong (don't trespass where there are private property signs or other warnings), so the most is someone comes out and tells you to move along. The actual act of painting just takes a few minutes, after that it's mostly waiting for the paint to cure.
I now own a home so I can paint in my backyard, or garage with the right precautions. But that wasn't always the case. When I was in college, I remember going to church parking lots during the week when no one was around. No one ever stopped and gave me any grief. Of course that was many years ago so today things may be different, but just an idea to think about.
Good luck,
Tony