Visible colours are really just a small part of what we call an electromagnetic spectrum. The majority of the spectrum is invisible to use, be we are able to detect it in other ways. For example, at one end of the spectrum, is radiowaves – which we use for radio transmissions. On the other end is X-rays and gamma-rays, which we detect using scientific and medical equipment. The visible spectrum lies in the middle of these, and ranges from violet, to green, to red. Our eyes can only perceive a small amount of the total electromagnetic radiation.
As strange as it may seem to us, there are colours out there that we can’t see and our perception of the what we define as colour is also limited. A Sean says, the colours we see are simply different wavelengths of a huge spectrum. Dogs can see many more shades of grey than we can. And insects see what we think as yellow as a really vivid pink colour. Amazingly, the red cape that matadors use to get bulls to charge are pointless – the bull is colour blind. It charges because of the movement of the cape, not the colour. I wonder whether a bat “sees” in colour?