Outdoor plastic mushroom and andronack chairs need to be brightened up. What type of paint should I use
I would use spray paint if you can. Just read on the can what it can be used for.
There are a lot of plastic spray paints available now
This person used Rustoleum spray paint and recommended using a satin finish for a longer lasting finish. https://www.heytherehome.com/spray-painted-plastic-outdoor-chairs-update