Add a couple of format c/f and c/b to emit colours.

tmux.1

@@ -6694,6 +6694,12 @@ results in
 replaces a
 .Nm
 colour by its six-digit hexadecimal RGB value.
+If an argument of
+.Ql f
+or
+.Ql b
+is given, it will instead produce the SGR escape sequence to set the foreground
+or background colour respectively.
 .Pp
 A limit may be placed on the length of the resultant string by prefixing it
 by an