GMS 2.3+ Changing the opacity of a Room layer in the room creation code


I have an instance layer that is only in the beginning of the game that needs to have its opacity adjusted. It contains objects that are elsewhere in the stage, but the object (instances) itself needs to remain at 100% opacity for every other instance it is used in the game, therefor it's easiest to make this one layer semi-transparent in the creation code of the first room. How would I code that? Is it possible with an instance layer?