def _Colour(parser, propObj, propCls, propName, propHelp, shortArg, longArg,
    alpha=True):
    """Adds an argument to the given parser for the given :class:`.Colour`
    property. See the :func:`_String` documentation for details on the
    parameters.
    """
    if alpha:
        metavar = 'R', 'G', 'B', 'A'
        nargs = 4
    else:
        metavar = 'R', 'G', 'B'
        nargs = 3
    parser.add_argument(shortArg, longArg, help=propHelp, metavar=metavar,
        type=float, nargs=nargs)
