Well, we know most of that already, but I have made a deduction. See, light travels in different wavelengths (you can ignore this term if you’re not into science). It merely means red color appears red because it’s the way our eyes resemble the physical form of light on that particular object.
I have a theory.
Let me make it simple:
The Eighth Color – a scientific theory.
Rainbow is seven-colored, VIBGYOR we call it (Violet, Indigo, Blue, Green, Yellow, Orange, and Red). We see them only after rain or high humidity in the atmosphere. It is actually just white sunlight passing through water droplets in the air. The drops (in the sky) act like a prism where lights get through and split into seven colors.
In simple words, there are only specific numbers of colors a human eye can see (7,000,000).
Everything that is not made of that color is simply invisible to us. So if you paint a wall of some mysterious color not recognizable to our eyes, be ready to take responsibility if someone walks into it and breaks their nose .
Light has a wavelength, like anything else in this world, which is in waveform (like sound). Which means it travels in space (vacuum or air-filled) at a certain speed or phase (angle). We can only see a range of that wavelength.
Say, red color lies in that wavelength (just for the sake of understanding). Then if a color that is similar to red somehow, but does not fall in the visible wavelength, is invisible to us.
For example, Bluetooth or wi-fi signals exist. But you cannot see them. Maybe a buffalo or cow can if that signal falls into their visible spectrum. Perhaps there are objects like a pencil or your hat, that is invisible to some creatures. You get the basic idea.
Update: It came to mind in around 2011. And I wrote this article originally in 2013 for www.rightside.in (so it is outdated).