The X11 window system provides a number of predefined colors,
stored in a file called rgb.txt.
The colors are defined
as an RGB triple and a name.
There are over 600 named colors,
although 100 of them are shades of gray.
This web page
allows viewing the X11 colors sorted by a variety of color models.
You can look
for a thorough explanation of the different color models.
The display is 100 colors per page, each color swatch is
a button containing both the name and the hexadecimal RGB triple.
Clicking on a color button will display a page whose background is
the color of that button with the alphabet displayed in both
black and white foreground colors.