def normalize_rgb(r, g, b, a):
    """Transform a rgb[a] color to #hex[a].
    """
    r = int(r, 10)
    g = int(g, 10)
    b = int(b, 10)
    if a:
        a = float(a) * 256
    if r > 255 or g > 255 or b > 255 or a and a > 255:
        return None
    color = '#%02x%02x%02x' % (r, g, b)
    if a:
        color += '%02x' % int(a)
    return color
