Mana washing

Mana washing is a slang term used by Magic R&D to describe effects that allows the player to spend mana as though it were mana of any color/type.^[1] It is most commonly used to allow players to pay costs from cards that weren't initially in their deck but they have gained access to.

Rules

From the Comprehensive Rules (June 7, 2024—Modern Horizons 3)

609.4b If an effect allows a player to spend mana “as though it were mana of any [type or color],” this affects only how the player may pay a cost. It doesn’t change that cost, and it doesn’t change what mana was actually spent to pay that cost. The same is true for effects that say “mana of any type can be spent.”