For out-and-about, a small palette to which I added extra half pans does the job. It's filled with 24 Daniel Smith watercolors (I have tubes). However, I've seen the idea of a portable mini-palette in several places and I wanted to try out my Derwent Inktense pencils.
Just scribble hard on a card. I covered mine with deli paper (stapled on the left side), but, wax paper would work also. The card holds 18 colors (cool colors on the other side)--beats carrying around 18 pencils in your purse!
And then I tested it out while running errands. It's magic!