Casein - the closest paint to digital art
I just wanted to espouse my new favorite medium. If you're a concept artist, coming from digital art, or are just wanting an alternative to gouache, I'd highly recommend trying casein paint.
Back in the Rockwell days, casein actually used to be one of the, if not *the* most popular paint for illustrators. It even competes with oil paint in finish/handling (see John Berkey's portfolio for examples). It lays down like gouache - you can do washes or opaque passages. You have a window to scrub/rework the paint if needed. Unlike gouache, you can glaze and layer with ease. After around a couple days, casein will cure and be even stronger than many oil paintings.
I started with oil, then went to gouache. When I eventually left digital art behind and decided to go all traditional, it's very frustrating to lay down my large studio paintings with gouache. Casein pretty much solved all those problems for me, and it's extremely cheap in comparison.
If you've heard of casein, I'd definitely go try it. I always thought it was some weird boutique medium until I actually started using it.