Abrelatas

[Tomisimo]

The best way to think about the imperfect and preterit is the imperfect is used when you aren't specifying or don't want/need to stress a starting/ending time for the action. When you do want to specify the starting or ending time (or simply the fact that it has ended), then you use the preterit

Fui pelirojo = I was red-haired (for a time, but not anymore)
Era pelirojo = I was red-haired