setting array element value for defined property

I feel a bit stupid because I think I might be placing a silly question:
how do I set a [nth] element value of a custom property defined as an array?

Example:

obj.defineProperty("colors", new Array()) ;
...
obj.setProperty("colors", "red")[n] ;
or
obj.setColors("red")[n] ; ?


Thanks.

obj.setProperty("colors", "red", n);

or

obj.setColors("red", n);