Once you determine the cost to cast Fungal Colossus, you may activate mana abilities to pay that cost. If the number of differently named lands you control changes while activating mana abilities (probably because you sacrificed one or more lands), the cost to cast Fungal Colossus remains what you previously determined.

Once you announce you’re casting a spell, no player may take actions until after the spell has been paid for. Notably, opponents can’t try to remove lands you control from the battlefield at that time.

Fungal Colossus’s mana value doesn’t change no matter what the number of differently named lands you control is.

To determine the number of differently named lands you control, count each land you control once, but only if its English name isn’t exactly the same as another land you’ve already counted this way.