Positional.
In my pen cup I have paper tubes. This FIXES the location of all of my pens. So the relative position of the pen in the cup tells me what color of ink is in the pen. My black ink pens are together, my green ink pens are together. And I have a particular ink order; blacks, greens, blue, reds.
Similarly I have a pocket protector that I carry my carry pens in; greens, grey, blue. In this case the pen color also matches the ink; green ink pens are green, and the blue ink pen is blue.
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