Theory: King Boo's Design in Luigi's Mansion

King Boo’s different design in the Luigi’s Mansion games is due to his mad emotions towards the Mario Bros. in said games.

Abstract
King Boo has become a staple Mario character over the years since his debut in Luigi’s Mansion. There has been much confusion within the community however over whether or not the same King Boo appears from game-to-game.

While a previous High Consensus Theory addressed King Boo’s differing design in Super Mario Sunshine, this theory hopes to explain King Boo’s different design between the Mario games and the Luigi’s Mansion games.

Arguments

 * Argument 1: King Boo has a very different design in the Luigi’s Mansion games. He has pink eyes with a dark “shadow” behind them, as well as a crown with a red/pink coloured gem on top. In the Mario games, King Boo’s appearance is that of a large, but otherwise, normal-looking Boo that has a golden crown with three small jewels embedded within. This design change goes unexplained between games while these characters are supposed to be the same person.


 * Argument 2: While another theory suggests that the gem in King Boo’s crown is the cause of King Boo’s design shift, this theory is debunked as of Luigi’s Manson 3. In the ending of Luigi’s Mansion 3, King Boo’s Crown’s Gem is removed and it vanishes after King Boo’s capture. Later on, when we see him in the Vault/Gallery, he retains his Luigi’s Mansion design, minus the crown. This proves the crown is not the cause of the design change.


 * Argument 3: The colours of various Boos’ faces are shown to change based on their mood in the Mario franchise. When Boos become shy when looked at in Mario platformers, their faces often change colour to red or purple.


 * Argument 4: There is an enemy encountered in Super Princess Peach called “Mad Big Boo”, which bears a striking resemblance to the design of King Boo in the Luigi’s Mansion games. Throughout the Luigi’s Mansion series King Boo is very mad, especially compared to his more lighthearted appearances in the Mario series, getting consistently more mad as the games go on. This would match the depiction of Mad Boos in Super Princess Peach perfectly.