The mini, mini absolute least number of pastels to take on a drawing hike are the following six Unison pastels:
These colors are designed to offer the bare minimum choice of hue, value and intensity for drawing the landscape, and "mixing" colors by blending and open-layering is required.
My little cigar box with this kit is pictured above. If I have room in my box, I add Grey-18. Also, since my little cigar box has the space, I add an eraser, a few pencils and some hard pastel grays. Tissue paper makes the cushion.
Now consider your medium-sized plein air palette. Illustrated is my two layer cigar box. The second tray is one I made out of Fome Core and glue, and is lifted out with a broad ribbon.
This is what I wrote in a previous post about my medium sized kit:
Since I am not in my big studio during my remodel, I am working in the house with my plein air kit. I make my kits from shallow cigar boxes. This one is a PADRON (Nicaraguan, Hand Made) cigar box: 6.5" x 11.25" x 1.75". The shallow depth allows for security of the sticks, and I find that wood's gentle touch is the kindest to my pastels. I notice that few cigar shops actually sell their empty cigar boxes, but those kind ones that do, get my return business. Cardboard is also gentle; plastic transfers too much shock, IMO. For extended trips (air travel), I add a sheet of thin foam bought at an upholstery shop, or scrounged from other places.
Further, I have constructed a tray out of foam core, with foam core dividers glued in place with Elmer's (children's) glue. A ribbon allows me to get the top tray out. When I do go (rarely) out the door, I put a couple of extra long rubber bands around the box.
When I went to Italy, I knew to add some yellows for facades, umbers just because and tile reds for roofs. You may wish to pick some colors that remind you of your painting locale. Mostly, I follow the artist's idiom that says, "If you get the values right, it doesn't matter what the hues are". You'll see in the photo above that I have six values represented, with an assortment of colors chosen almost by random. I try to stay away from too dark, and too light of vales, since this works best for "naturalist" works. Try to select tinted grays when you do choose gray.
If you have a burning desire to make your color statement, then you may finish your work in the studio where all of your colors are at hand.
If you absolutely must finish your work on-sight, then you'll need a big tray of pastels at hand. We'll cover the big tray next time, and then we'll go on to the easels that I choose for outdoor use.