Have you considered that what you're trying to do is impossible? Without constraints on the colors being sorted, there is no single "obvious" mapping from a ra

Have you considered that what you're trying to do is impossible? Without constraints on the colors being sorted, there is no single "obvious" mapping from a random collection of colours chosen from 3-space into 1-space. If you do have constraints on the colors being chosen then you'll need to be explicit as to what they are, and then use them in a non-trivial manner.