Well, I poked around at that a little bit and came up with:
shape main
{
setpen(0,255,0,1); // (R,G,B)
setfillcolor(0,255,0); // (R,G,B)
rectangle(0,0,100,0);
}
Which does get me a green line, but there is a sort of gray ghost line alongside of it. The explanations aren't too extensive in the help...