Cascading Style Sheets - HTML and XHTML: The Definitive Guide - page 456

url( service :// server.com / pathname )
With CSS2 properties, the keyword url is required, as are the opening
and closing parentheses. Do not leave any spaces between url and the
opening parenthesis. The url value may contain either an absolute or a
relative URL. However, the URL is relative to the stylesheet's URL, not
necessarily the document's base URL. This means that if you use a url
value in a document-level or inline style, the URL is relative to the HTML
document containing the style document. Otherwise, the URL is relative
to the @import ed or <link> ed external stylesheet's URL.
8.4.1.5. Color property values
Color values specify colors in a property (surprised?). You can specify
a color as a color name or a hexadecimal RGB triple, as for common
HTML/XHTML attributes, or as a decimal RGB triple unique to style prop-
erties. Both color names and hexadecimal RGB triple notation are de-
scribed in Appendix G .
With CSS2, too, you may assign just one hexadecimal digit instead of
two to the red, green, and blue (RGB) components of a color. That digit
is simply doubled to create a conventional six-digit triple. Thus, the col-
or #78C is equivalent to #7788CC . In general, three-digit color values are
handy only for simple colors.
The decimal RGB triple notation is unique:
rgb( red , green , blue )
The red , green , and blue intensity values are decimal integers in the
range 0 to 255, or integer percentages. As with a url value, do not leave
any spaces between rgb and the opening parenthesis.