def next_color(color_text):
    """
    Given a color string "#xxxxxy", returns its next color "#xxxx{xy+1}".
    """
    hex_value = int(color_text[5:], 16)
    if hex_value == 255:  # ff
        return color_text[:5] + "fe"
    else:
        return color_text[:5] + "{:2x}".format(hex_value + 1).replace(" ", "0")