Just so I get this precisely right: This makes it so Sisay's Ring only adds one mana anymore, but of any color, right?

That is the intent, but the wording (and the typo) were pretty rough. It's still not perfect, I think.

Well, mana abilities are active effects, so I would suggest:

"If enchanted player would add any amount of colorless mana to their mana pool, instead that player adds one mana of any color they choose to their mana pool."

(Template from Spectral Searchlight.)

That's closer to my original wording.

Are you sure mana abilities are effects? I'm pretty sure most of them are activated abilities.

An activated ability is an effect. What I meant was that they use active voice, not passive.

Okay, sincewe are talking about templates and the intricacies of wording here, let's be precise:

• Abilities do not equal effects.
• Abilities create effects.
• Effects can make us perform actions.
• Actions in turn can have effects.

Mana abilities by definition are abilities - specifically the term refers to certain activated abilities as wellas certain triggered abilities triggering from mana abilities themselves (which means it always starts with an activated ability).

That said this card presumably doesn't want to be restricted to mana abilities, but also include the effects of abilities that add mana to a mana pool but don't fall into the more narrow definition of a mana ability e. g. Soulbright Flamekin or Cathodion.

The only replacement effect using the word "add" seems to be on False Dawn, and this card's current wording mirrors the wording on that card extremely closely.