The web‑safe colors are the 216 colors used by browsers regardless of the platform. The browser changes all colors in the image to these colors when displaying the image on an 8‑bit screen. The 216 colors are a subset of the Mac OS 8‑bit color palettes. By working only with these colors, you can be sure that art you prepare for the web will not dither on a system set to display 256 colors.
If
you select a non‑web color, an alert cube appears
next to the color rectangle in the Adobe Color Picker.