color darkgrey is not a color value

Hello,

I received the following error for "a:visited": "Value Error : color darkgrey is not a color value : darkgrey darkgrey" while attempting to validate against CSS level 3.  The relevant CSS is:

a:visited {color: darkgrey;}

However, as far as I know "darkgrey" is a valid keyword.  Is this an error in the validator, or am I missing the obvious?