Glossy reduces the absorptive surface area considerably, so would reduce passive solar gain; however - remember you'll be eliminating the cooling effect of water that evaporates as it passes through the container walls, so it's sort of a compromise.

Al

Do you know if water passes through all terracotta pots? I might paint the top halves white or something,
but their 5-1-1 mix got up towards 90 degrees on the surface today, which scares me when it is still only 70 outside!!

Being on cement does not help.

Real terra cotta is porous. Even though it may look smooth and impermeable, it is definitely porous, unless it's glazed. I never worry about the unglazed terra cotta orange pots... only darker ones, like black plastic or the dark green ones.

I keep my large terra cotta pots on a concrete slab in front of a garage all summer with no ill effects. Everything grows splendidly. It's just my opinion, but I wouldn't worry so much about the clay pots... only darker colored ones.

Jodik, what climate are you working with? We get a few weeks of spring then straight to mid 90s with 100+ not uncommon. Generally people do not consider containers around here in the heat, but I have very little ground space that is already packed!

I'm in Central Illinois... our climate runs the gamut, from dry and frigidly cold, occasionally dipping below zero with wind chills to match... to summers that are quite hot and humid, occasionally entering triple digits. Not very often, mind you, but we do hover in the 90's long enough for pot color to matter somewhat, depending on where it's placed.

I do a lot of growing in containers on a large concrete slab in front of a garage. I use either unglazed clay, or white plastic, although I have successfully grown in black pots. I think the porosity of the clay is what helps the roots to breathe, and it helps dry out the medium in a timely manner, so more is needed, which in turn cools down the plants.

I don't know that I'd want to block the pores of the clay, myself, by painting them... but it's just my opinion.