The white-painted walls are a type of radiant barrier technology, which uses reflective coating to keep radiated heat where you want it. I'm currently finishing up installing a radiant barrier in my attic, which will reject a lot of the radiant heat from the sun in this upcoming summer. It's great technology, and not all that much more expensive than painting your walls - I got mine for about $0.10 per square foot. It's basically large rolls of aluminum foil backed with a fabric mesh for structural support - very light and easy to work with.
Last summer, I only had a portion of it installed, and I found myself retreating under that portion for a lot of the day because it felt so much cooler (and this was in the formerly hottest part of the house).
Traditional blown-in insulation only blocks convected heat; the radiated heat (like you feel when standing in the sun) goes right through it, heating you and your house and costing money and carbon to remove.
The downside of the technology is that you lose some solar heat gain during the winter when it helps you, but here in Texas where I live, that's far less of a concern than surviving 5 months of 100 degree heat and blazing sun. Plus at least some of the house interior's heat will be reflected back down during the winter, rather than radiating out into space on those frigid clear nights.
If you live somewhere hot and sunny, you _really_ need to look into this stuff. My partial installation last summer paid for itself in a single year in savings. I look forward to seeing how a full installation does this summer. Save money, save carbon, be more comfortable!

Here is an article about an environmentally friendly insulating paint additive. It helps reduce heating and cooling losses and helps deaden sound along with other benefits.

http://www.hytechsales.com/insulating_paint_additives.html

This would make your walls reflect heat and cool air more effectively than bright paint alone.

I think painting exterior walls white would be better thermally, but wonder whether selected darker walls with added thermal mass (positioned across from south-facing windows) would be better for maximizing winter passive heat gain.

Regarding radiant foil barriers in the attic, I've always wondered how well these work and play with moisture issues (which may vary seasonally) and wooden joists that hold up my roof deck. I want to avoid any condensation or mold issues... Last summer I added a solar powered attic fan in order to help reduce summer daytime heat gain.

Dave and Fiver,
Radiant Barriers are incredible products when installed appropriately. As to moisture issues, we carry a laser perforated product that can't trap moisture. The important part is making sure you have a minimum 3/4" air gap on one side. We roll it out on top of attic insulation. It blocks ~ 95% of the radiant heat. And, that's heat trying to come inside in summer, and radiant heat that's escaped from the ceiling in winter.

As to darker thermal mass for passive solar gain. Probably easiest to do on floors. You can see a net-zero retrofit I did for Discovery Channel on Greenovate.

For a climate specific roadmap to net-zero, visit www.NewLeafAmerica.com

I always dismissed this as a myth but I recently painted my bedroom white and it has made such a huge difference. I’m planning to pay a local Denver painter to paint a lot of rooms in my house white or cream colors.