I've often been asked what supplies I use to paint on location and the most important item would be my Judson's pochade box. This one accomodates 2-5x7 panels along with a small set of Golden Open acrylics. They take quite a while to dry so painting in the Rocky Mountain winds or bright Badlands sunshine is much easier.
Right now my colour palette includes titanium white, ultramarine & manganese blue, cadmium primrose & indian yellow, cadmium red light & alizarin crimson and sap green but even though they are only 22 ml each it's a little tight in the box so I may adjust that eventually. I also use a small rag (one of my husband's cut up t-shirts), a small round & small flat brush, a brush washer (metal cups that attach to my palette) and a view finder as well as a pen and pocket moleskine for thumbnail sketches.
The pochade is still nice and light when everything is all packed plus I bring a bottle of water and granola bar along with my phone for photos. And that's it. I really like to keep things light and simple.