Many times the Agency or Studio wants a photo for the casting. Whatever it is, dress in costume. Look like the character you want to play.
If they are casting Extras for a sporting match, get a cap or a jacket, look like a fan. If they are looking for Extras for a street protest, look like a protester.
You have to be able to get the right image to those Casting as quickly as possible.
You got to walk the line between being ‘unique’ and blending in.
In one way you want to be average and match the environment, but you want something to catch an eye so that you are selected.