Imagine you're looking at wall with 3 colored circles, red green blue - something happens and you can now see a fourth circle that wasn't there before.

You've experienced it already... by shining a black light on ultraviolet pigment.

It's the same principle behind color blind test patterns: https://cdna.allaboutvision.com/i/eye-exam-2017/color-blind-...

A color blind person sees one color - a normal vision person sees two.