There are currently three primary unlockables in Cuphead. These modes and features can be unlocked after completing the necessary qualifications.
Expert Mode is unlocked after a completed Normal playthrough of the game. Cuphead cannot be fully completed in Simple Mode. This mode will increase the difficulty like the name suggests and add health to the boss, making each boss harder to defeat. (including mini bosses) The difficulty will be selectable before boss fights. Beating a boss with a "A+" rank like in normal mode will instead grant you a "S" rank.
Two-Strip Mode can be accessed by defeating most of the bosses with an A grade or higher on normal difficulty. Talk to Forkington in Inkwell Isle Three to unlock the feature. The visual style can be toggled in the options menu.
Black and White Mode
Black and White Mode is a secret mode where everything turns black and white. This mode is slightly more difficult due to colorless objects, making parry objects harder to spot.
To gain this mode, complete all six of the Run 'n' Gun stages without killing a single enemy. A successful clear will grant the player a grade "P", and the white flag will update with a P upon a successful completion. A turtle at the docks of Inkwell Isle Three will unlock the mode when being spoken to after clearing all six stages. The visual style can be toggled in the options menu.