Waterproof Watercolor Palette
This is a small, compact and waterproof watercolor pallette. Although you can carry a small amount of watercolors in any size container - an altoids tin, for example, a waterproof palette can't be beat because you can open and close it anywhere, and not worry about spilled paint.
Here is my Martin Watercolor Palette. (Review on Amazon)
Also, I collect different sizes of drinking straws, which are great in protecting your brushes in travel. I keep the brushes right in the pallette to shave weight and space, and I seek out shorter brushes, which are easier to stow while traveling. While I was in the Alvord Desert, I used a single brush for all my notes.