216 = 6 * 6 * 6 -- That's a "Color Cube": a 6x6x6 3D cube of 216 equally spaced colors. Not necessarily the colors you'd actually want, though, just mathematically convenient. Figuring out the closest color in the cube to any color is quick and easy (so you can do a quick 24=>8 error diffusion dither, for example, which needs to do that every pixel), but lots of the colors suck.

Web-Safe Colors (a Color Cube)

https://www.peachpit.com/articles/article.aspx?p=23671&seqNu...

Not to be confused with a Time Cube.

https://en.wikipedia.org/wiki/Time_Cube