Hello judges. I recently ran into a question and I heard different opinions on it (from different level judges).

The scenario is: Alvus casts Emrakul, The Promised End, Ninfadora casts Pact of Negation targeting Emrakul. The Pact resolves. Alvus finishes their turn and start controlling Ninfadora's turn. Ninfadora controls 4 Steam Vents and 1 Island. Alvus call a judge and ask:

Hello judge am I allowed to, in response to the Pact trigger, tap all the Steam Vents generating red mana?

Is it possible? I am asking because the opinions I received are split in:

• No, you cannot tap for red mana unless you have a cost requiring red mana
  
• Yes you can do that because any time a player has priority he/she can activate mana abilities (605.3a)
  

Thank you.

Yes, you can do that. The game does not impose limits on the amounts or kinds of mana players can produce, and definitely not any requirement that you be able to spend mana in order to produce it. If for some reason you want to produce a billion blue mana in a deck that's completely incapable of ever spending blue mana (and can find a way to do it), then go right ahead.

So yes, since Alvus controls Ninfadora, he can force her to tap all the Steam Vents for red mana when she receives priority with the Pact trigger on the stack.