I prefer a logical method, meaning there being a reason for the color changes.
My example would be brighter is higher (as in heat).  I always find it easiest to explain a reason for my choice

Voltage         Positive          Negative
5                    Green           Black            Left as listed
7-9                 Blue             Black             lighter blue I believe
12-16             Red              Black            COMMON
24                  Yellow           Black             lighter
48                  White            Black             lightest

--
Rob KB8RCO