The number in a circle format means “mana of any color,” whereas the curvy-diamond [?] shape is specifically colorless mana. You can use colorless mana to pay a mana-of-any-color cost, but you can’t use colored mana to pay for something that requires colorless.
![[SOS] Petrified Hamlet](https://mtgzone.com/pictrs/image/ec87d960-d81e-4978-9ad7-5e1f32fafdff.webp){kind=link}
The number in a circle format means “mana of any color,” whereas the curvy-diamond [?] shape is specifically colorless mana. You can use colorless mana to pay a mana-of-any-color cost, but you can’t use colored mana to pay for something that requires colorless.
Ok I quit before anything required colorless mana.