Apparently, there are only 216 of the 256 colours that won't be altered in some strange way by various browsers. Here are the colours that you can be relatively sure will look the same from browser to browser.
| 216 Hexadecimal Colour Codes | |||||
|---|---|---|---|---|---|
| #ffffff | #ffffcc | #ffff99 | #ffff66 | #ffff33 | #ffff00 |
| #ccffff | #ccffcc | #ccff99 | #ccff66 | #ccff33 | #ccff00 |
| #99ffff | #99ffcc | #99ff99 | #99ff66 | #99ff33 | #99ff00 |
| #66ffff | #66ffcc | #66ff99 | #66ff66 | #66ff33 | #66ff00 |
| #33ffff | #33ffcc | #33ff99 | #33ff66 | #33ff33 | #33ff00 |
| #00ffff | #00ffcc | #00ff99 | #00ff66 | #00ff33 | #00ff00 |
| #00ccff | #00cccc | #00cc99 | #00cc66 | #00cc33 | #00cc00 |
| #33ccff | #33cccc | #33cc99 | #33cc66 | #33cc33 | #33cc00 |
| #66ccff | #66cccc | #66cc99 | #66cc66 | #66cc33 | #66cc00 |
| #99ccff | #99cccc | #99cc99 | #99cc66 | #99cc33 | #99cc00 |
| #ccccff | #cccccc | #cccc99 | #cccc66 | #cccc33 | #cccc00 |
| #ffccff | #ffcccc | #ffcc99 | #ffcc66 | #ffcc33 | #ffcc00 |
| #ff99ff | #ff99cc | #ff9999 | #ff9966 | #ff9933 | #ff9900 |
| #cc99ff | #cc99cc | #cc9999 | #cc9966 | #cc9933 | #cc9900 |
| #9999ff | #9999cc | #999999 | #999966 | #999933 | #999900 |
| #6699ff | #6699cc | #669999 | #669966 | #669933 | #669900 |
| #3399ff | #3399cc | #339999 | #339966 | #339933 | #339900 |
| #0099ff | #0099cc | #009999 | #009966 | #009933 | #009900 |
| #0066ff | #0066cc | #006699 | #006666 | #006633 | #006600 |
| #3366ff | #3366cc | #336699 | #336666 | #336633 | #336600 |
| #6666ff | #6666cc | #666699 | #666666 | #666633 | #666600 |
| #9966ff | #9966cc | #996699 | #996666 | #996633 | #996600 |
| #cc66ff | #cc66cc | #cc6699 | #cc6666 | #cc6633 | #cc6600 |
| #cc33ff | #cc33cc | #cc3399 | #cc3366 | #cc3333 | #cc3300 |
| #ff66ff | #ff66cc | #ff6699 | #ff6666 | #ff6633 | #ff6600 |
| #ff33ff | #ff33cc | #ff3399 | #ff3366 | #ff3333 | #ff3300 |
| #ff00ff | #ff00cc | #ff0099 | #ff0066 | #ff0033 | #ff0000 |
| #cc00ff | #cc00cc | #cc0099 | #cc0066 | #cc0033 | #cc0000 |
| #9900ff | #9900cc | #990099 | #990066 | #990033 | #990000 |
| #9933ff | #9933cc | #993399 | #993366 | #993333 | #993300 |
| #6633FF | #6633CC | #663399 | #663366 | #663333 | #663300 |
| #6600FF | #6600CC | #660099 | #660066 | #660033 | #660000 |
| #3300FF | #3300CC | #330099 | #330066 | #330033 | #330000 |
| #3333FF | #3333CC | #333399 | #333366 | #333333 | #333300 |
| #0033FF | #0033CC | #003399 | #003366 | #003333 | #003300 |
| #0000ff | #0000CC | #000099 | #000066 | #000033 | #000000 |
To convert the hexadecimal numbers into decimal R-G-B numbers:
Hexadecimal 00 is equal to decimal 0
Hexadecimal 33 is equal to decimal 51
Hexadecimal 66 is equal to decimal 102
Hexadecimal 99 is equal to decimal 153
Hexadecimal CC is equal to decimal 204
Hexadecimal FF is equal to decimal 255
Example: #ff0033 is 255-0-51 in RGB
Note that the hexadecimal colour code is always preceded by # (number sign) and should always be enclosed in quotation marks when used in an HTML tag. With stylesheets (CSS), the colour code is not enclosed in quotation marks but the hexadecimal number must still be preceded by # (number sign). You will probably want to use hexadecimal colour codes for bgcolor (CSS: background), text (CSS: color), link (CSS: a:link) - the colour your links will be before the links have been selected; Alink (CSS: a:active) - the colour a link will be as it is being selected; Vlink (CSS: a:visited) - the colour the link will be after it has been selected; Other (CSS only, not fully supported by all OS) (CSS: a:hover, a:focus) - the colour the link will be when the cursor is hovering overtop. It is a good idea to specify a background colour similar to the background of your background image if you are using one; occasionally the image does not load.
What do the numbers mean? Briefly, they are based on RedGreenBlue. The first two numbers/letters signify how much red is in the colour; the middle two numbers/letters how much green; the last two numbers how much blue. Remember that the colours being mixed are not pigments but light (like the colours cast from a prism) so the traditional pigment colour wheel does not work.
 This is how the colours mix with light.red + blue = magenta red + green = yellow green + blue = cyan red + blue + green = white |
If all the colours are used fully (signified by the letter "F"), the light becomes white. (Remember when mixing pigment, all colours used fully yields the colour black.) If none of the colours are used (signified by the number "0" zero), there is no light and so it becomes black.
By mixing all the colours (red, blue and green) equally, one gets white, grey or black:| #ffffff | #cccccc | #999999 | #666666 | #333333 | #000000 |
By mixing red and blue equally and eliminating green, one gets a shade of magenta:
| #ff00ff | #cc00cc | #990099 |
By mixing red and green equally and eliminating blue, one gets a shade of yellow:
| #ffff00 | #cccc00 | #999900 |
By mixing blue and green equally and eliminating red, one gets a shade of cyan (turquoise):
| #00ffff | #00cccc | #009999 |
I'm still trying to get a handle on what the other numbers and letters mean but you can see how easy it is to get the primary and secondary colours now.
| "#ff0000" is Red | |
| "#00ff00" is Green | |
| "#0000ff" is Blue | |
| "#ffff00" is Yellow | |
| "#ff00ff" is Magenta | |
| "#00ffff" is Cyan | |
| "#000000" is Black | |
| "#ffffff" is White
|
As you experiment with colours, you'll see that some of them don't show up well against light or dark backgrounds. 
Please bear this in mind as you choose colours for your HTML documents. I have also made a little colour palette (gif 2.77kb) that you can use if you are making your own gifs.
internet safe 216 colour chart . other "named" colour chart . horrible colour combo demo
© llizard 1998-2003, 2005
May 2005: I FINALLY corrected the misspelling of 'hexadecimal'. I can't believe I had 'hexidecimal' there for so long!