Standard CSS color keywords are limited to 149 named shades; the hexadecimal (or “hex”) color method has access to the full RGB gamut. The key to learning hex is understanding the hexadecimal counting system.
The CSS named color system is notoriously bad: keywords are often difficult to remember (navajowhite), illogical (darkgrey actually displays lighter than dimgray) and/or visually questionable (lime and fuchsia verge on the bilious).
Until CSS custom named hues and variables are widely supported, preprocessors remain the best way to create your own custom color names. Defining site colors in Sass creates a cohesive color library that can be used to style content quickly and easily, with named colors that are significantly easier to remember and type than their hex equivalents. However, it can be burdensome to build a decent color library with a logical naming system.