Auto paint is a baked on acrylic enamel, if properly cured it would not be toxic, but I don't know why any paints would be needed for turtles or their environment.
Painting a turtle itself is more harmful and pointless than anything.
you want to be very careful with the type of paint you use. Most paints of toxic materials making them up, I would go to a local pet store and see what they have to offer
Putting paint on a turtle is a bad idea by itself. No to make it worse that your using car paint. But, if you do more research you might be able to find special paint that won't harm a turtle. Good luck!
Most reptile specialist pet shops near me sell dedicated turtle paint. The proper stuff can be more expensive but your turtle will thank you for it. Don't take risks with their wellbeing.