A small handful of colored pencils are great for adding a bit of life to a sketch. As mentioned previously, I often sketch with a single pencil in a dark color (or an interesting one) and then either add watercolor or let it stand as is.
Because colored pencils are a dry medium, they’re simpler and more immediate than some other materials. There’s no need to carry brushes or water into the field, and once you put down the color, you’re done. You can normally use them in museums where wet media are prohibited, as well.
Cases make carrying several colored pencils in the field easy (while protecting their points), but you can also just bundle a handful of them together with a rubber band. If you’re trying to keep it light and simple, that’s a good option.
You can choose from traditional wax-based pencils or the newer oil-based pencils. I would suggest buying a similar color of each type from open stock to try them out. Wax-based pencils generally won’t lift or smear under watercolor washes and, therefore, are one of my favorite tools. Oil-based pencils may be harder and drier, and they smear more easily.
For easy maneuvering, bundle your pencils up with a rubber band, as Joseph did here.
I like to simplify my kit for on-the-spot work. Having only the primary colors and a few supplemental pencils works just fine. The combination of colored pencils and ink is a happy one.
Try a simple ink framework for a landscape, then add color using only a few colored pencils. Layer for more complex colors.