If you're riding on a train or don't have enough time to do an elaborate study of a scene, there's still a fun thing one can do: fetching the colours of your view in small patches.
Over the time you can thus gather a whole library of colour mixtures that you can later use in final works.
This collection of patches came from my view out of the train, while driving back to Berlin.
The scenery was pretty clear with a bit of dusk above the horizon, open fields were changing with small patches of forests.
I found that hint in Felix Scheinberger's "Wasserfarben für Gestalter" ("water colours for designers")