Set the stroke color of drawing operations.
YourScreen.stroke(r, g, b)
YourScreen.stroke(color)
Nothing
YourScreen.beginDraw();
YourScreen.clear();
YourScreen.stroke(255, 0, 255);
YourScreen.noFill();
YourScreen.rect(0, 0, YourScreen.width(), YourScreen.height());
YourScreen.endDraw();