Because when Crowley's in charge of designing his little angel cosplay, he dresses like an utter dweeb.
But Aziraphale really just took Crowley's outfit and shifted the hue 😂
"You're going to have to do it, Angel."
(wriggles fingers) "Make me look like one of your lot."
"You can't be serious. Heaven is getting emptier by the minute and besides, it's not like a disguise would really fool anyone."
"Last time Muriel said I looked like a hornet. A murder hornet. The last thing we need is to get swarmed by a bunch of fledgling angels eager to make a head start on the next celestial war."
"Oh for the love of- Fine." (he snaps his fingers dismissively at Crowley) "There you go."
"Angel you've barely changed a thing."
"Yes, I have!" (He pops a mirror into existence to prove his point) "Look at yourself."
"You only gone and changed the colour."
"I also changed the fabric."
"Ohhh don't even get me started on the gold... is this polyester?-"
(outraged) "How dare you. That's Tela aurea, you utter philistine!-"
"I'm the philistine? You've turned me into a ... vanilla version of myself."
"Well I think you look nice."
(Crowley looks back and forth between his outfit and Aziraphale. His brow furrows in a way that makes it clear he's squinting behind his glasses.)
"Did you really match my jacket to your tie?"
"Not even remotely, my tie is actually a shade or two darker and-"
"And these cheeky little streaks you've added to my hair, was that to symbolize all the stress you've caused me-"
"I'm sure I have no idea what you mean-"
(smirks) "-or are you just marking your territory, can't help but notice the color is seems pretty close to your own hair-?"
(brightly and frantically) "Dear me! Would your look at the time, we really should be getting a move on-"
"Are you afraid some other angel might make a play?"
(yelling now) "OFF WE TROT."