Filter posts by 2 categories

I use categories to filter down posts to a particular subject. I’d like to be able to then further filter down the results within a single category based on a second category set (or something equivalent). What’s the best way to do this?

For example, I’d like to be able to see just the blue roadsters by selecting one category from the tope set and a second category from the list below.

Category Set 1:
Roadster
Sedan
Truck

Category Set 2:
Red
Blue
Green

Thanks!