Move the mouse left and right to shift the balance. The "mouseX" variable is used to control both the size and color of the rectangles. 

def setup():
    size(640, 360)
    noStroke()
    colorMode(RGB, height, height, height)
    rectMode(CENTER)


def draw():
    background(0.0)

    r1 = map(mouseX, 0, width, 0, height)
    r2 = height - r1

    fill(r1)
    rect(width / 2 + r1 / 2, height / 2, r1, r1)

    fill(r2)
    rect(width / 2 - r2 / 2, height / 2, r2, r2)