Two events A and B are said to be independent if the fact that one event has occurred does not affect the probability that the other event will occur.
If whether or not one event occurs does affect the probability that the other event will occur, then the two events are said to be dependent.
In the given scenario, they want to be sure not to get the same dress.
Therefore, these events is dependent, because the second person's color of dress is dependent to the first person's dress