One of the things I love about “Contact” is that the contact mechanism chosen by the aliens was so close to what I guessed aliens would use when I first learned about SETI.

Decimal is not universal. Not seconds, not meters, not sound frequencies used for communication, not colors. Our sky isn’t blue, it’s purple. Ask any bee and they’ll tell you. But hydrogen glows at very specific colors and that only changes if you are moving fast enough.

The fundamental colors are the colors of the elements and, I might argue, their oxides. As reflected by light or when they incandesce. Gold. Rust. Arsenic green. Carbon black. Maybe the emission bands of noble gases, though those are hardly every day items.

(If I were a very clever alien though, and I discovered exotic states of matter where the elements behaved differently, and I only wanted to talk to other very clever aliens, I might use those instead to talk over the heads of the younger or dumber species, which is why I stopped contributing to SETI. We are looking under the wrong rocks, IMO).

The Aurora colors are similarly universal.

The most common green color from excited nitrogen molecules going back to normal by emitting 557.7 nanometer photons. Oxygen makes 650 nm red, and the 427.8 nm blue is from nitrogen ions.