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There are certain literary conventions about the representations of good and evil. In visual media they're often colour coded. The convention that evil characters wear dark colours and good characters wear light ones is so widespread that you're either going along with it or deliberately subverting it.
Honestly, either way is fine, depending on what you're going for. Playing into expectations can be a powerful way to establish things quickly and also highlight the significance of a character and have your players understand the situation they're in. Or... use those expectations to mess with player assumptions and pull a switcheroo just when they think they have it all figured out.
Commentary by memnarch (who has not seen the movie)
Poor BB-8. But such is the life of the Star Wars sidekick droids. For now, I'll bet that BB ends up taking a wrong turn or few, getting caught by a couple of officers, and proceeds to zap them all before rolling away in a panic. I'd have guessed the coin gun instead of the shock probe, but there has been a dearth of drunken aliens to provide a proper reload since the last firing. And DJ looked like he was taking all of the previous ammunition as well.
More surprising is the actual laundry room. If dressing as the enemy wasn't already a spy trope, I'd suspect that the writers liked the Episode I comic dialogue enough to invent a plausible scenario where this could potentially happen in the comic. These disguises seem to be a little lacking in the hidden identity department however. If anything is going to make things go wrong, it's going to be someone actually looking at the humans of Team Casino. Finn might be recognized as a probably former trooper, I don't think I've seen an obviously Imperial/First Order lady officer before, and DJ's still got that scraggly mustache.
I do appreciate that we skipped over how they actually boarded the ship, like the droid landing ships in Episode I or the Peace Moon entrance in Episode VII. There would probably be so many more plot holes noticed or things to be confused by if we saw that sequence. Having it simply be "Team Casino's on the ship now, deal with it" means we can get to lots of silly sneaking around. Straight fighting can come later once the humans have been recognized as intruders, or after the Raddus does its crash landing.
Transcript
GM: What’s Finn’s sub-party doing on the Supremacy?
DJ: We find a laundry chute.
GM: Because that’s never led to things going off the rails before... <roll>...
GM: Okay, sure. Then what?
DJ: Slide down to the laundry room!
DJ: First Order uniforms for everyone. Vestis virum facit.
Finn: Disguises. Nice.
Rose: BB-8, you need some camo too. A round, white droid won’t pass as evil.
Finn: Here, I’ll put this waste bin over you. Now you’re black and cubic.
[SFX]: chonk
GM: BB-8 can’t see anything.
Finn: Just follow the sound of our footsteps.
BB-8: I’m sure this won’t go hilariously wrong.
DJ: Hmm. Maybe we need to rethink this whole plan.