Web Safe Colors
The table below is commonly considered to be the "web-safe" color palette, developed at a time when many computer displays were only capable of displaying 256 colors. A set of colors was needed that could be shown without dithering on 256-color displays; the number 216 was chosen partly because computer operating systems customarily reserved sixteen to twenty colors for their own use; it was also selected because it allows exactly six shades each of red, green, and blue (6 × 6 × 6 = 216).
These days personal computers typically have at least 16-bit color and usually 24-bit TrueColor. The use of "web-safe" colors has fallen into practical disuse, but persisted in culture.