I like my sketching supplies to be as portable as possible, so I made a mini watercolor kit in an Altoids Smalls tin. Not only is it cute, it's very handy. I like using a Kuretake waterbrush pen with watercolor to make lines, mostly, so this works well for me.
I painted the inside of the tin with Rustoleum appliance touch-up paint, to keep it from rusting, and to have a white surface for mixing paint, if I ever feel like doing that.
I hot-glued the paint tins down and filled them with Winsor & Newton artists' water colors:
Permanent Alizarin Crimson
Between burnt sienna and viridian, I stuck down the yellow ochre that comes in the Winsor & Newton travel watercolor kits.