From the Super Mario Wiki
Kolorado's father, who appears as a non-living Dull Bones (a sub-species of Dry Bones) in Paper Mario: The Thousand-Year Door, was the father of Kolorado from Paper Mario. Although Kolorado's father is now a Dull Bones, he was once a Koopa.
Kolorado's father is seen in one of the early rooms of Hooktail Castle in Paper Mario: The Thousand-Year Door, where Koops mistakes the bones for those of his seemingly late father Koopley. Thinking that the rumor that Koopley is dead was finally confirmed to be true, Koops persuades Mario to read a letter that the skeleton was clutching. If the player chooses to indulge Koops' request, the heroes learn of Kolorado's father's attempt to destroy Hooktail. Although Kolorado's father managed to infiltrate the dragon's castle, he soon found himself "stuck", and expired soon after.
Despite failing to achieve his goal, Kolorado's father managed to learn much about Hooktail's tactics and weaknesses, which are described in his letter. He warns the reader that Hooktail tends to feign surrender when weakened, trying to catch opponents off guard by offering gifts in exchange for their mercy. He also claims that the one thing Hooktail fears is something "that begins with 'cr' and ends with 'icket'". The letter ends with the phrase "My last words go to my son, Kolorado: I love you, and I'm proud of who you've become." As these words are read, Koops realizes that these are not his father's bones after all, and Mario and his team continue onward. His bones remain intact and the letter can be read again at any point in the game. Kolorado's father plays no further part in the game.
If the player chooses not to read the letter, they simply move on and neither gain the insight about Hooktail's weaknesses and tactics nor the knowledge over the true identity of the bones.