I am making a custom knife for a game. After the humanoid dies I want to change the body parts material and color. I do not even know where to begin, or if it is even possible
Reshiram110 is correct. I'll provide you with a few links.
Died event Material BrickColor
You may also need a for loop, but it's not exactly necessary. You can use it to make every single BasePart in a character a certain color and material.
In case you want another explanation, here's how I would approach it. Using the Died event, I'd use a for loop to change the body parts material and color, but since you appear to be a beginner, you can write out each line for each body part.