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