Suppose I show you an emerald and ask whether it’s green or grue. It’s “grue” if it’s green today but will turn blue next Halloween.

Which is it? That’s the “new problem of induction,” according to philosopher of science Nelson Goodman. It’s a big problem: Scientists basically assume that the universe behaves consistently over time, but there’s no logical reason to expect this.

A more immediate usage: “Yed” is the color of a traffic signal when the last legal driver manages to get through the intersection. “The existence of the color yed is hotly debated in philosophy, and opposing viewpoints are often taken by traffic cops and vehicle operators.”