Here is the full list of all 27 Colourless achievements.
You found a secret!
Defeated first boss
You found all the secrets!
Game completed!
Unlocked Ninja costume.
Unlocked Knight costume.
Unlocked Monster costume.
Unlocked Soldier costume.
Unlocked Thief costume.
Unlocked Mike costume.
Unlocked Flamer costume.
Unlocked Tasko costume.
Unlocked Walker costume.
Unlocked Kenny costume.
Unlocked C for Colourless costume.
Unlocked KROW costume
Unlocked Jack Lope costume.
Unlocked Reina costume.
Unlocked GOW costume.
Unlocked C3LL3R costume.
Unlocked Muffin costume.
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Unlocked Boss Mode costume.
Unlocked Killer achievement.
Unlocked RIP achievement.
Unlocked Head crush achievement.
Unlocked PRO achievement.