I wonder that, too. I actually wanted to draw a Halloween themed picture with Red dressed as Papyrus and Blue as Sans, but it didn’t happen.
I can imagine Red being really serious about it, going up to an enemy like, “Surrender your arms or I’ll have to use force!!” I like to think Red takes video games seriously to a level of absurdity. I mean, in the illustration for 101% hard mode, he engaged unlimited form and knocked over his chair just because a game he was playing got hard. I like to think that’s why he’s not necessarily a gamer despite at least causally liking video games, because he knows they rile him up needlessly. I mean, he’d probably realize nothing in Undertale is really out to get him, so he’d probably approach it the way he did when he fought Wonder-Blue. He’d force them into submission even if it means dying, but I can’t imagine Red murdering Blue, he just wanted to beat him into submission. It would go down like the Papyrus fight, he and the enemy would beat each other up until one of them submits and they spare each other. Some enemies in Undertale can actually reach sparing point by just beating them till they’re weak, as opposed to using the “act” function. Still, I imagine he’d come across the Whimsum enemy, see that it can be spared immediately, and try approaching the remainder of the game with a softer heart knowing some enemies aren’t enemies at all.
Blue on the other hand would inevitably end up on a neutral route. I can imagine he might get frustrated trying to spare some enemies and just say, “Hey bro, you wanna die? I wanted to let you live but you had to be stubborn, so be it.” Blue would probably end up killing one of the main characters like Undyne, realize he did something real screwed up, and reset. Like, ever see the way Undyne dies when doing a pacifist or neutral run? It’s really sad and drawn-out. He’d probably end up resetting at least once, although hesitantly because he doesn’t want to put-up with going through the beginning of the game again, but would be pleasantly surprised that the dialogue actually changes a little after resetting.
Pink would definitely go for a pacifist route. She has a really strong motherly instinct that’s real cute, but if the enemies giver her a hard time, she’d probably mash the buttons angrily like, “I’m going to spare you and you’re going to let me!!” And, when she finally does spare them, she’d be like, “See? That wasn’t so hard, was it sweetie?” I can imagine her getting all emotional during the ending sequences, too.
Wonder Black would go like, “…”
Wonder Green would probably play the pacifist route just for the experience but not for any moral reason. He’d go on to do a genocide run, and a soulless pacifist run because he’s probably the kinda kid who’d play a game dry to see every last detail it had to offer. He might need a few hugs along the way though.
Wonder Yellow and Wonder White would do pacifist runs, no doubt. Yellow might even pick the game back up just to see the characters again and say hi, but only by opening his old save file where they’re all waiting for Frisk to pass through the last door. He’d end up exhausting their dialogue, and certainly would not reset the game.