colors names versus accessibility

Hi all,

It would be great if somebody could clarify the following:

Section 9.1 of WCAG10-CSS-TECHS 
(http://www.w3.org/TR/WCAG10-CSS-TECHS/#style-color-contrast) 
says to use numbers, not names, for colors.

Section 4.3.6 Colors of CSS2.1 Specs 
(http://www.w3.org/TR/CSS21/syndata.html#color-units)
says that there are 17 colors keywords (color names. right?)

So, why avoid colors names if they are legal according CSS21 specs?
How does colors names CSS value can have an effect upon the contrast? 

Maurício Samy Silva
http://www.maujor.com/