Does not require any additional mods to work!
Rename a carved pumpkin in an anvil to "Color" Impostor, for example - Red Impostor.
Starting from snapshot 25w03a, it became possible to create custom items without using mods.
Unfortunately, there's still no regex support (meaning you can't, for example, write "red impostor" instead of "Red Impostor", or "123 red impostor 123").
If the .zip doesn't work, extract the files from the .zip into a folder.