Hi prev! Okay so Green Gaster genuinely has a shocking amount of evidence for it. I believe it comes from the - unfortunately quite good actually - 'DELTARUNE 67 THEORY: Why Gaster is Green and in Between' (yes that is actually the title and yes it is relevant to the theory). But it's expanded a lot since then, and there's a lot more evidence than in that video, so I'll mention what I remember!
Basically, while Gaster has still only ever been depicted with grey-scale colors, a lot of definitely or maybe Gaster related things are green. But the main chunk of the theory revolves around the surprisingly common appearance of something green being between something red and blue.
In the Undertale 10th Anniversary stream, one of the changes was that Gaster's door was the same color as the one's in sans and Papyrus' house. People noted that the gap where the door would go was replaced with a painting of a bone with a green background. On the left is Papyrus' door, on the right is sans'. On the ground floor, there's the red joke book on the left, the green couch (which gold falls into and gets lost ooo Gaster bullshit), and the blue-ish grey pet rock on the right which is maybe a little weak but stands better when next to other evidence.
In Deltarune Chapter Three, the doors to the A, B and C Rank rooms are a tall red door on the left, a medium green door in the middle, and a short blue door on the right. The green door of course leads to the prize ball machine, which has some major foreshadowing regarding Dess and Kris' hand, and the ball machine disappears after you get this secret.
In Snowdinn, Papyrus' puzzles have red circles, blue X's and green triangles, which go pretty much unused (forgotten).
Other evidence for green without the whole red and blue thing includes the Chapter 1 save select menu and the battle box both being green (If you don't know why this is Gaster-related, see 'Device Theory' by MollyStars - basically the assets for several parts of Deltarune, including the title screens, save selects, goner maker and game over screen are named in a completely different style to everything else in the game, e.g. IMAGE_GONERBODY, which is similar to SURVEY_PROGRAM, which we are almost certain was made by Gaster. The idea is these things are part of an in-universe device made by Gaster for the purpose of allowing us the Player to interact with the world of Deltarune.)
Spamton's strings are green, the green crayon is missing in Kris' house, the Old Man's battle text says 'The air crackles green with freedom.' (There's a lot of interesting parallels between Gerson and Gaster, two brilliant people in their fields who were 'swallowed up' by their creations and replaced by Alphys. This is also different to Spamton and Jevil's 'The air crackles with freedom.') The chapter 3 Forgotten Man room is entirely green, and can be reached through a battle with green enemies.
Also, as an extra bit, there's a set of old deleted tweets from Toby Fox talking about the idea of Luigi being erased from Mario canon, which starts with "luigi takes the fall". Luigi's Mansion is now " 's mansion", and is just an empty house forever. Waluigi now just Wa . Guy associated with green falls and is erased from reality.
That's the basics but there's genuinely so much more, the best place to read it all is this thread by Morkie.
https://xcancel.com/morkanslil_y/status/1971182462467490177#m
As for Pirate Gaster? A lot less evidence, not even really a theory, more of a bit. Someone, I can't remember who, said 'Hey what if Aqua becomes pirate themed in chapter five because her name means water' then 'hey what if Gaster was a pirate wouldn't that be crazy'. Everyone liked that so he's a pirate now, and any Gaster-adjacent characters (and Aqua because she was here first) are his crew.
'Evidence' for pirate Gaster includes:
- He makes 'Vessels' (boat reference)
- Darkness often described similarly to water
- NormalNPC (the blue and yellow bird lightner that is the basis for G-follower 3 and maybe killed Onion??) repeats things like a parrot
- Chapter 5's dojo OST is pirate themed (some say they can hear Gaster's Theme in it but I don't, and even if I could, that's like saying two rooms MUST have been decorated by the same person because they both have Ikea furniture, those notes are everywhere)
- It's really funny actually
So Gaster is Green, In between and a Pirate.
Now that I've actually written it down it all sounds insane. I hope some of that helps a little!
