Returns the inverse (negative) of a color.
    The red, green, and blue values are inverted, while the opacity is left alone.

def _invert(color):
    """
    Returns the inverse (negative) of a color.
    The red, green, and blue values are inverted, while the opacity is left alone.
    """
    col = ColorValue(color)
    c = col.value
    c[0] = 255.0 - c[0]
    c[1] = 255.0 - c[1]
    c[2] = 255.0 - c[2]
    return col