This tool/toy/demo is a way to navigate all 16 777 216 colors in the RGB color space. You are navigating a 256x256x256 cube, and based on your position, the background color is calculated. You can navigate the space just like you would navigate any 3D space in a videogame. Move your mouse to look around and use wasd to walk. You can also press E to go up, and Q to go down, and hold shift to move 5 times faster. Essentially, the more west you are, the more red you add. The more south you are, the more green you have. And the higher you are, the more blue component is in the color.
Well, colors are 3 dimensional. Most of the color representations have 3 components (like RGB, HSL or HSV). A picture is just 2D, so it is not really possible to nicely show a 3 dimensional entity such as color in it.
This page is best viewed on desktop Chrome. It probably won't be operable on mobile browsers, and probably will be slow on Safari.