No matter what type of paint you're using, painting up a color chart of all the colors you've got gives you an easy, at-a-glance visual reference.
Photo © Marion Boddy-Evans. Licensed to About.com, Inc
This watercolor chart hasn't aged very well over the past 20-odd years. It's faded and the unevenly painted swatches become even more evident. I wrote the name of each color in pencil beneath each swatch. These originally all went from dark to light in tone, but some light tones have faded completely.
The brown paper tape
I used to stretch the sheet of watercolor paper before I painted the chart is still evident on the sides. I never trimmed the edges of the sheet, nor framed it; it has always lived on a shelf, ready to be pulled out for consultation as needed.