Ok, so, what do you mean?
Do you want to me to this:
// top left
worldrenderer.pos((double) 100, (double) 100, (double) 0).color(1.0f, 1.0f, 1.0f, 1.0f).endVertex();
// bottom left
worldrenderer.pos((double) 100, (double) 200, (double) 0).color(0, 0, 0, 1.0f).endVertex();
// bottom right
worldrenderer.pos((double) 200, (double) 200, (double) 0).color(0, 0, 0, 1.0f).endVertex();
// top right
worldrenderer.pos((double) 200, (double) 200, (double) 0).color(1.0f, 0, 0, 1.0f).endVertex();
or how do you want me to do it?
EDIT:
I got this to work, but it makes a diagonal line from the top-right vertex..
// bottom right
worldrenderer.pos((double) 200, (double) 200, (double) 0).color(0, 0, 0, 1.0f).endVertex();
// top right
worldrenderer.pos((double) 200, (double) 100, (double) 0).color(1.0f, 0, 0, 1.0f).endVertex();
// top left
worldrenderer.pos((double) 100, (double) 100, (double) 0).color(1.0f, 1.0f, 1.0f, 1.0f).endVertex();
// bottom left
worldrenderer.pos((double) 100, (double) 200, (double) 0).color(0, 0, 0, 1.0f).endVertex();
// bottom right
worldrenderer.pos((double) 200, (double) 200, (double) 0).color(0, 0, 0, 1.0f).endVertex();
// top right
worldrenderer.pos((double) 200, (double) 100, (double) 0).color(1.0f, 0, 0, 1.0f).endVertex();