So let’s say this Alien eyes see on the infrared level, he would see normal RGB like you and I, but could be still know there is colors there? What reason is there for the alien to discover there is colors there?
I think I can’t imagine how we discover something that we don’t see, or mostly that there is something that I don’t know
Light is an electromagnetic wave. Humans don’t naturally perceive most of the electromagnetic spectrum: https://en.wikipedia.org/wiki/Electromagnetic_spectrum
So, your theoretical primitive alien without technology wouldn’t know, but one with sufficient technology level would know. Assuming that it’s not an ignorant one.
So the reasoning behind would be: Hey our Space eyes are wave receiver>Hey, there is more waves then that> Hey thrrr is this part that shows this thing that I can see but alien on the planet earth would be able to see 🤔?
If they evolved to perceive certain wavelengths, they will see those wavelengths. The wavelengths that they haven’t evolved to see could be observed using specialized tech.