Wouldn't it make sense for there to be such an option? What happens if your workflow has it so that a photo is done being worked with when it has no color label? Otherwise it is still in process and needs more work.
A color label is obvious as is no color label. But no color label is more unique than trying to distinguished a color label. In other words, seeing no color at a glance is simpler than trying to remember or figure out what a particular color means.
Scott