I know that for xterm on my box, the default colors don't use the
whole 0 to 1000 range. White is set at R=680, G=680 B=680 or some-
thing like that. I always assumed that they reserve the difference
between 680 and 1000 for 'bold', but, that's just a guess. I know
for some non-xterm terminals, white is R=1000, G=1000, B=1000.
I think you could get a decent match by just multiplying
the decimal (0-255) values in the wikipedia table by 3. That'd
put you from 0 to 785. That way, you split the difference
between white=680/680/680 and white=1000/1000/1000 terminals.
The true answer to your question on how you match those colors
in the table exactly is going to be different for each terminal
or terminal emulator, I think.
-Mike