If you schedules are set in such a way that they overlap, then the schedule currently applied will always be overridden by the start of the new schedule.

For example, if you have scheduled a room to change to blue from 3pm to 5pm, then revert to its previous status, and you create a second schedule to change the lights to red at 4pm, then the lights will first change to blue at 3pm, then to red at 4pm.