Lower volume products tend to be priced higher. That is if the main product is white but they then produce a black one, then the black one will be higher because volume pricing for the different components (black items) will be a tiny bit higher
Demand, people will want something slightly different so a black one will be priced higher, i.e., economics, 101, demand vs.supply.
Because Apple wants too, simply as that, it may not cost them any different, it may not be demand and supply but rather apple using higher pricing to evoke a higher premium experience and/or just make more money.