Thank you. I was confused since I always see colorless mana as a number inside a grey circle...
A gray circle symbol with a number on it is no longer used to mean colorless mana, but now just means the stated amount of mana of any type (C.R. 107.4b). For example, 1 means one mana of any type (C.R. 107.4b). Some cards formerly had the text "Add 1 to your mana pool". In general, those cards now say "Add C" instead. Similarly, "Add 2 to your mana pool" is now "Add CC". However, this doesn't affect 1 in costs. For example, the 1 symbols in Bearer of Silence don't become C instead, since they appear in costs. See also this thread and this thread.
EDIT (Feb. 14, 2021; Jul. 20, 2021): Edited.