I am currently using a dark green 30 year fence stain. The solar gain in the late fall before they are wrapped seems to help. It was $5 at the Habitat for Humanity Store.
The best method I have found for painting is to place a 8' iron pipe on two saw horse. You can paint all the edges and outside of a super without having to rest any wet paint on the ground. They can dry in the air and then are ready for a second coat.