I like hearing about people with full size black crew cabs in Phoenix complaining about how bad their AC is.
White walls will help a tiny bit on reflecting radiant heat from inside the house, less than with foil, but won't help much with heat transfer from the outside. That's why insulation is needed, although foil and air layers can be effective, one layer that gets dirty much less so. As I recall 'space suits' are something like almost 20 layers of air/foil layers.
During a winter camp years back a friend an I experimented with using foil. We built a crude semi-circular foil fence around the camp fire, and had to move back from the fire and let it die down a bit. It was very effective.
For cold weather try reindeer skins :^) I recall a documentary where a gentleman traveled thru Siberia in the winter and stayed with some of the nomadic locals. They used reindeer skin yurt like tents, and then made the guest a small cubical reindeer skin tent within the larger one. They had footage of hime describing how comfortable it was even though he was just in light clothing for sleeping, and it was something like 50 below outside.