These 216 six-digit hexadecimal color codes define what are
considered safe, non-dithering colors. This is sometimes called the Netscape palette because Netscape (Web browser maker) defined this palette to display color graphics based on an 8-bit system (28 - 40 (reserved by operating system) = 216 colors). The goal was to have a common set of colors for many kinds of computer displays and operating systems (Macintosh, Windows, Unix). The importance of these safe colors has been reduced dramatically over the years as display monitors have become more capable to display many colors. 16 or 32 bit color systems are common (16 bits means 216 = 65,536; 32 bits means 232 = 4,294,967,296).
These safe colors are presented here for their historic and reference interest.
Notice that all safe colors are Hex Triples.
If the monitor is an 8 bit one, a color not in this safe list will dither by adding pixels of two different colors from this safe list to the same area to try to approximate the color you want. This dithering causes the colored area to appear speckled.