Low energy

High energy

Color is determined first by frequency. Let's start by determining what a typical person would see when looking at electromagnetic radiation of a single frequency. Physicists call this monochromatic light. (The literal meaning of this word is "single color", but the actual meaning is "single frequency".)

Low frequency radiation is invisible. With an adequately bright source, starting somewhere around 400 THz (1 THz = 1012 Hz) most humans begin to perceive a dull red. As the frequency is increased, the perceived color gradually changes from red to orange to yellow to green to blue to violet. The eye doesn't perceive violet so well. It always seems to look dark compared to other sources at equal intensity. Somewhere between 700 THz and 800 THz the world goes dark again.