It's the flitetest brown "water resistant" foamboard. That and Adams Readi Board are considered the go-to brands. They're exactly the same weight, lighter than any others. I would think the FT brown would be stronger and heavier, but I weighed them both, and they do in fact weight the same. The brown is impregnated with silicon or some waxy compound. Water resistant is great but you might have trouble with painting. (There are ways around any trouble you might have.) The Adams is just $1 and also works great. Some prefer Adams, some FT brown. I keep both.
There's always talk of other products that are almost as light, etc. but if you're able to get one of these, these are are the ones you want.