I ran an experiment on soil temperatures. I took two black containers that each contained a citrus tree. I painted one container white, and left the other container black. I took soil temperatures at different times during a sunny day.

TIME.................WHITE CONTAINER ............BLACK CONTAINER

10:30 AM..................79F......................................83F
1:15 PM...................86F.....................................102F
2:15 PM...................88F.....................................105F
3:00 PM...................89F.....................................108F
5:30 PM...................88F.......................................99F

Citrus roots greatly stop growth at 95F, so for the tree in the black
container it lost almost a full day of growth. Figure out how many days of growth are lost during the three sumer months, when the soil in black containers get to hot. Many times if you pull a tree from a black container and look that all the roots growing on the side of the container that faces the sun (hot side) you can see where the roots have died from the heat. This experiment was done on September 5, 2006. The temperatures during June, Just and August would be much higher. - Millet

Bov, this was done in our area years ago. I do remember the exact temperature but the blacks pots were quite a bit higher. If the temp is 100 degrees what would the pot temp be?

Hey Millet, thanks for taking time to do this! Since you mentioned spray painting pots a lighter color for temp control (couple yrs ago) , I did it. Only on one side...& when pots are in sunroom in winter, I leave the black part exposed to the sun to help keep roots warm.
This summer we had a scorture in July & I felt how warm the soil was by putting my finger in it...very WARM. And, it was sprayed off-white (as you saw in pics I posted).

Patty, when I painted my container white, I only painted one half of the container white. I left the other half of the container black. This way, just like you, I can turn the white side south in the summer, to help keep the soil cool, and the black side south in the winter to help in heating the soil. High temperatures in containers are frequently the cause of poor growth of nursery stock during summer. Media in black containers exposed to the direct sun may reach temperatures in excess of 130F (55C) during mid summer or early fall. However, do not use a ready made white container, because white containers are translucent, algae grows on the outer surface of the medium, reducing the amount of oxygen available for the roots. Alway use a black container and paint it white. Light cannot then penetrate to the soil. Inspection of root development in black containers exposed to the direct afternoon sun reveals few, if any, roots on the sunny side of the container, but at the same time, roots on the shaded side are healthy. - Millet

I have often thought about painting pots with thermochromic paint which changes from black to white at 27C. This is now available in the uk from
http://www.mutr.co.uk/pdf_files/SMARTCOL.pdf
Large quantities would be expensive.
A google search for 'Thermochromic Pigments' would probably find some US sources.
Perhaps Millet can do some scientific tests with this paint to see if it's worth buying!?

This is incredibly useful. Thanks for sharing your research.