I collect colors on index-sized cards and use them later to select pastels and paints that already are harmonious. By doing this, I struggle less when it comes time to pick out the colors I want to use when starting a painting.
Collecting colors from nature or even works from other artists, and paintings I admire is easy. The most difficult task is to remember to set aside time to do the collections. I started doing this after a workshop with Kathleen Newman in Chicago, an urban sketchbook workshop. She showed how she collects colors on water color scraps. I tried the watercolor cards but found I use pastel so much that the collections of colors means more or is easier to use if the card collection is also in pastel. Whatever medium you use most, seems to make more sense to me. That makes picking out the sticks, or tubes if you use tubes, of color easier for me.
Growing your Color Collection takes the Guesswork out of Color Mixing
Over time, if you set aside time to collect colors when you are painting en plein air, or in the studio during down time, your collection will grow. It seemed pretty puny when I started after Newman's workshop, but now is large enough for me to put into a box of their own.
Since mine are pastel and not watercolor, I put them into acetate sleeves that are about the size one would use for note cards, 4.5x5.5 inches. These are available from www.clearbags.com
What I learned
Be patient, the collection will grow
The colors save me lots of time and cut out all the trial and error since they are collected from nature and already are harmonious.
I found I have favorite color combinations so after a while I don't need to collect as many color sets on cards as I did when I had no frame of reference, or when I started.