Rainbow Mode

Rainbow mode or Rainbow Run is a game mode added in the A Farewell to Arms Update, activated by talking to the NPC Bowler, who must be rescued from a cell in the Gungeon after the High Dragun has been beaten at least once.

Effects
Bowler gives the player a Rainbow Chest at the start of each floor, which can spawn items of any quality. The player will be able to grab one of the items, and only one. This is the only source of items throughout the run, and most other items are removed:
 * At least one and one  item is guaranteed to spawn. The remaining items have an approximate distribution of 12%, 41% , 23% , 12% , and 12%.
 * Items in shops, on pedestals and from The Lost Adventurer and the Gunsling King are all removed.
 * Bowler will sit on all non-rainbow chests, preventing the player from collecting items. Note that this means if the player finds a brown chest or similar that Bowler is not sitting on, then it is a natural rainbow chest and can be safely opened.
 * Any non-rainbow chest opened will appear empty, and Bowler will leave a note saying that he took the item.
 * Using a key on a non-rainbow chest will consume a key with no payoff, which can prevent access through the Oubliette and Locked Doors, such as a lock having a chance to appear at the Old Crest's room. Other than those instances, Keys are not needed much in Rainbow runs.
 * This includes chests from Winchester's game, the mirror room, the Truth Chest, rat chests and paid Synergrace's synergy chests.
 * Keys are still consumed if the player tries to open a locked chest.
 * Pickups, Lies, Junk and Gold Junk dropped from destroyed chests remain collectible, but other items retrieved from breaking a chest will be replaced. This includes Ser Junkan,  unless  the player has not unlocked him yet.
 * If a chest is destroyed, there is a chance that a note will appear from Bowler that he took an item, and no Junk or pickups can be obtained. This replaces things like tier Guns that can drop in normal runs.
 * If the chest or pedestal is a Mimic and the player kills it, Bowler will leave a note instead of an item.
 * In co-op, Bowler will sit on revival chests, but they will still revive the dead player if opened.
 * Shrines like the Companion Shrine or gift reward from the Dice Shrine will not work, but negative effects like a heart container removal from the Companion Shrine will still happen if interacted with.
 * Spawned chests will be immediately replaced by a note.
 * Mirror Rooms are completely useless as the chest cannot be destroyed to obtain Junk.

Exceptions

 * Master Rounds still appear normally.
 * Bowler doesn't take quest-related items, such as the Busted Television or Obsidian Shell Casing.
 * Bowler also cannot take the Bullet That Can Kill The Past.
 * Bowler cannot take the Gnawed Key.
 * Junk will never be taken. This includes Lies, Gold Junk, and Ser Junkan (if he's not previously been unlocked).
 * Rainbow runs are especially useful to The Robot because of this, as his power increases if he collects junk.
 * Items are still dropped by the Resourceful Rat during his punch-out phase.
 * Munchers and the Witches function as normal.
 * Rainbow chests found naturally still contain items, all eight of which can be picked up.
 * Using a Shortcut will still provide a selection of three guns, in addition to the starting level's Rainbow Chest. You can take as many of the three guns as you're usually allowed on that floor.
 * Spice will still have a chance to replace pickups after being consumed as normal.
 * Knight's Gun will still dig up items as normal.
 * Spraying the Demon Face with a water gun will still drop an item.
 * Items that spawn upon clearing a room like Glass Guon Stones are still obtainable.
 * However, chests that spawn on room clear will be taken.
 * While most beneficial Shrines don't work, the Glass Shrine will still give the Glass Guon Stones, and the Y.V. shrine will still give its buff.

Trivia

 * The special Bowler pose is likely a reference to Solaire from Dark Souls' "Praise the Sun" pose. This is further reinforced by how the chest bursts into flames.
 * When talking to him in The Breach, Bowler will mention that he wants to get rid of the Ammoconda in Rainbow Runs.
 * Bowler signs his notes with the following aliases:
 * The Rain Train
 * Rainbuddy
 * Bowbow
 * This run type was created by YouTuber Retromation.

Bugs

 * There is a visual bug that can cause Bowler to appear behind the chest he is supposed to be sitting on.
 * There is also a visual bug where Bowler will not sit on top of Rat Chests, but will be positioned on the floor as if he were sitting on a smaller chest.
 * There is a glitch in co-op mode where the two players can grab two different items at the same time. If the glitch is successful, the players should have the items they took, along with 2 other items in the chest on the floor that they can pick up. The players have to pick up the items at the exact same time for this to work.
 * Dropping Rainbow Loot obtained this way has a chance to cause it to disappear.
 * Occasionally, if timed right, if one player grabs the item on the right, and the other player grabs the top-right item, the majority of the remaining items will actually remain. (Usually all items remain.)
 * If a player collects an active item with an active item currently held, the previous active item may disappear.
 * Despite the fact that most chests spawned as room rewards are replaced by a Note while a Rainbow Run is active, Synergy Chests are not affected by this and will appear as normal, though they will still not give an item.
 * If Rainbow Mode is activated, talking to Bowler yields two options. Selecting “Just wanted to chat.” causes the old avatar of your character to appear if you are playing as one of the four main characters. If playing as The Bullet, the avatar is instead a black square. This doesn't affect The Robot, The Paradox or The Gunslinger.