On the rim of every watercolor palette of mine you will see a T an S or an O next to each well written with a Sharpie. At a quick straight down glance, this little guide will tell me if the color is Transparent, Sedimentary or Opaque.
This is a great way to learn your palette. To learn and remember which colors are Transparent, Sedimentary or Opaque. And if you can't remember, the letter code is there.