def contrast(color, center, mult):
    """Expand the color values by a factor of mult around the pivot value of center.

    color: an (r, g, b) tuple
    center: a float -- the fixed point
    mult: a float -- expand or contract the values around the center point

    """
    r, g, b = color
    r = (r - center) * mult + center
    g = (g - center) * mult + center
    b = (b - center) * mult + center
    return (r, g, b)