One of the hardest concepts to explain to people is how Kiss My Brass is beneficial to so many hair shades. I think people generally understand why Kiss My Brass works on blondes: simple color theory. Purple cancels gold because they are opposite on the color wheel.
It gets a little more complicated when you introduce multi-colored or brown hair into the mix, in the form of ombre gradients or highlighted brown hair. The violet pigment does not affect the portion of the hair, other than imperceptibly deepening the hue a tiny bit to make hair look thicker.
I actually love this trick on black hair. Layering a violet hue over pure black makes the color pop in the light and makes it pop and gloss.
Of course, KMB also works to brighten natural grey and maintains creative or "fashion colors" like lavender, purple and silver hair.