#from some Gtk2 code
my $hex = sprintf( "0x%02x%02x%02x40",
     $radj1->value,   # 0..255
    $gadj2->value,   # 0..255
    $badj3->value,   # 0..255
);

print "$hex\n";
# something like 0x3cb37140 appears 
# and is what the fill-color-rgba wants below
# but it claims it's not numeric

 $hex += 0x1;
 $hex -= 0x1;
 my $resp= ($hex) ? "$hex is numeric\n" : "$hex is not numeric\n";
 print $resp;

$rect->set('fill-color-rgba' => $hex );
# claims $hex is not numeric