COLOURS
Primary/Secondary Colours All light is composed of 3 primary elements (blue, green and red) from which all other "colours" are derived by mixing greater or lesser amounts of each element. Most objects absorb some (or all) of one or more of the primary elements of light, reflecting only the remainder which we see as a "colour". Grass appears green because chlorophyll (present in all plants) completely absorbs the red and blue elements of light, and only reflects the green element. Another 3 secondary colours - yellow, magenta and cyan (or aqua) - are each composed exclusively of only 2 of the 3 primary colours. Thus "yellow" is composed of "green+red" (ie there is a total absence of "blue"), "magenta" is "blue+red", and "cyan" is "blue+green". These 3 secondary colours of light are often called the "primary colours of paint" by artists and oil-painters. White and grey are "colours" of luminosity, composed of greater or lesser degrees of an equal mixture of all 3 primary colours. Black is the total absence of all primary light, ie. it has no "colour" at all because all 3 primary colours are "missing".
Hexadecimal Colour Values Colour names (eg. "navyblue", "copper") vary from browser to browser, but colour values do not. The BODY BGCOLOR attribute, the FONT COLOR attribute and the 4 text-colour attributes (TEXT-LINK-ALINK-VLINK) all take rgb values in the form #rrggbb. Each r, g and b value is a hexadecimal value between 0-255. Black is #000000 and white is #FFFFFF. Any value with identical rr,gg,bb parts is a shade of grey. Thus dark blue is written as "#00007F" (7F=127 or half-value) while light blue is #0000FF (FF=255 or full-value). Mixing colours is an additive process - the larger the value, the more light goes into the final colour. Using <FONT COLOR> attributes, a 16-colour screen chart is written as:

The actual number of video-colours available depends on the operating system in use. The original "Windows 16-color" standard is a reflection of the basic 4-bit operating filesize, having 2⁴ (=16) bits of information. The next filesize up, 8-bits, enables 2⁸ (=256) allowable colour values, and a 16-bit file gives 2¹⁶ (=16 million) possible variations.

Only 16 basic pre-defined colours have names universally recognised by both Netscape and Explorer browsers. They are:

Here's a 27-color chart based on 3 x3 x3 combinations of "00", "7f" and "ff", with some of their "accepted" font descriptions:

000000-III 00007F-III 0000FF-III black, navy, blue
007F00-III 007F7F-III 007FFF-III green, teal, slate blue
00FF00-III 00FF7F-III 00FFFF-III lime, spring green, cyan/aqua
7F0000-III 7F007F-III 7F00FF-III maroon, purple, med.slate blue
7F7F00-III 7F7F7F-III 7F7FFF-III olive, gray, (?),
7FFF00-III 7FFF7F-III 7FFFFF-III med.spring green, (?), (?)
FF0000-III FF007F-III FF00FF-III red, (?), magenta/fushsia
FF7F00-III FF7F7F-III FF7FFF-III (?), (coral), orange?
FFFF00-III FFFF7F-III FFFFFF-III yellow, (?), white

"What's in a name?..."
The difficulty of using descriptions is immediately illustrated by the fact that none of the 27 "balanced" colours seem to match any of the following names (which when used throw up other colour-shades entirely): brown, chocolate, cream, crimson, indigo, khaki, x-lemon, magnolia, ochre, orange, pink, scarlet, sienna, tangerine, vermilion, violet...