How do I get a black plot on a white background. I can get about any color on a with background, except black. 'plcol0 (0)' does not work. Why???

I'm using plplot (5.6.1) and perlDL. Here's my sampl code:

use PDL;
use PDL::Graphics::PLplot;

$x = sequence 6;
$y = $x ** 2;

plsdev ("xwin");
plscolbg (255,255,255); # Gives white bg
plinit ();

pladv (0);
plvpor (0.15,0.85,0.1, 0.9);
plwind (0.0,5.0,0.0,26.0);

plbox (0,0,0,0,"bcnst","bcnst");
plcol0 (0);  # Does NOT give black, just gives nothing...
plline ($x, $y);

plend ();