$cr->set_source_rgb (0, 0, 0);

##</code><code>##

$cr->set_source_rgb ($red, $green, $blue)
           $red: double
           $green: double
           $blue: double

##</code><code>##

 $cr->set_source_rgba ($red, $green, $blue, $alpha)
           $red: double
           $green: double
           $blue: double
           $alpha: double