INDEX CARD #26:
COLOR ON THE WEB (26a)
A 24-bit number is used in the RGB system.
(In base two, a "0" or "1" is referred to as a bit. Thus, "24-bit" can be loosely interpreted as a base two number that uses 24 digits.)
The first 8 bits tell how much red the number contains; the next 8 are for green; the last 8 are for blue.
Using the binary name for each number is a bit unwieldy, so most people use the hexadecimal representation instead (use the "group-by-fours" technique):
MORE ON COLORS (26b)
#000000 is black.  #FFFFFF is white.
Combining equal amounts of red and blue gives purple:
Lots of colors have names, as well as numbers. Be careful, though: not all color names are supported by all browsers. The table on pages 7982 of the Weasel Book lists the named colors.
Here's a word that shows up a lot when you start investigating color on the web:
What does "dithering" mean? To "dither" means to mix pixels of available colors in different dot patterns, to create the illusion of a new color or shade. Some samples are available in Worksheet #26.
Printable version of Index Card 26a
Printable version of Index Card 26b