(Face Customization): Mention hex color specs.

man/custom.texi

@@ -566,10 +566,15 @@ before the attribute name indicates whether the attribute is
 attribute by invoking that field.  When the attribute is enabled, you
 can change the attribute value in the usual ways.
 
-  On a black-and-white display, the colors you can use for the
-background are @samp{black}, @samp{white}, @samp{gray}, @samp{gray1},
-and @samp{gray3}.  Emacs supports these shades of gray by using
-background stipple patterns instead of a color.
+  For the colors, you can specify a color name (use @kbd{M-x
+list-colors-display}) for a list of them) or a hexadecimal color
+specification of the form @samp{#@var{rr}@var{gg}@var{bb}}.
+(@samp{#000000} is black, @samp{#ff0000} is red, @samp{#00ff00} is
+green, @samp{#0000ff} is blue, and @samp{#ffffff} is white.)  On a
+black-and-white display, the colors you can use for the background are
+@samp{black}, @samp{white}, @samp{gray}, @samp{gray1}, and
+@samp{gray3}.  Emacs supports these shades of gray by using background
+stipple patterns instead of a color.
 
   Setting, saving and resetting a face work like the same operations for
 options (@pxref{Changing an Option}).