I have been playing around with a new medium lately, Holbein's Acryla Gouache. I really like the way it handles, it behaves like traditional gouache without all the tedious traditional parts. It doesn't shift in color and tone when it sets, I can layer more than i remember being able to with gouache, and it dries waterproof. The individual colours can be used transparent with the colour remaining fairly brilliant, or opaue in an easy way.
Colour mixing can be a bit challenging trying to get just the right amount of water and pigments floating around on your palette, and I think it would be nice to have a wider range of tubes available. The paints are a bit expensive for the tiny tubes, but I find a little pigment goes a long way. Obviously this medium travels a lot better than oils, and I think I might have just found my new plein-air medium.
Here is a couple of little tests I did, a landscape from a photo I had laying around from Muir Beach and a quick portrait done at a workshop at school.