Back in the olden days, the off-brand Irish Spring rip-offs didn't have the marbling all the way through -- they were a bar if white soap with a thin layer of greenish color. Knifing the real Irish Spring showed off that the special fancy coloring went all the way through the bar.

You're welcome.

Don't step on the Irish love of knifin', though. That'll only cause trouble.

It's not just coloration. Irish Spring's gimmick is that it uses more than one type of deodorizer, as indicated by the different coloration.