Important to note that this post was written with acrylic in mind, watercolors have their own palette choices that work differntly from acrylics. I'll address watercolors in a future post.
Why is palette choice so important to painting en plein air? Ultimately you want to carry the minimal amount of equipment to make the picture happen. You could carry every color you have available but that would make for a very heavy pack and awkward hiking. Using every color available tends to leave a painting with a disharmonious feeling. Paintings tend to have a harmonious feel when colors have been mixed with other colors in the painting. When you using paint straight from the tubes paintings tend to not jive as well as if you’ve added a touch of another color in your palette to the other colors.
The important thing with an en plein air palette is the colors you get by mixing 2 and 3 colors together. Some people use the same palette over and over again and others are constantly rearranging their palette, dropping in a new red or blue or swapping out a yellow. To each their own.
In acrylic I tend to work with the same palette for a warm season and switch to a more expansive palette in the winter months. This is because I usually work mostly inside in the fall and winter whereas in the summer I tend to work on location far more often.
Currently my palette is considered pretty large by plein air standards and it’s highly likely I’ll get rid of a yellow and a blue shade, but we’ll see.
Here are some excellent links on the subject of limiting your palette.
Channeling Homer also has a "pamphlet" about palette picking, and it's good
I frequently limit my palette to 2 or 3 colors, usually titanium white, a blue and then a warm color like titan buff or yellow ochre. This is an exercise I may explore again.