Reading about the rulings for Gilded Lotus, I've discovered that "add three mana of any one color" means: add three mana of a single color, not three mana of possibly distinct colors.
Black Lotus' text reads exactly the same. This means the widely known one-turn win consisting of Fireball, Channel and Black Lotus could not possibly work, since Black Lotus alone could not provide {R}{G}{G}.
Is this actually correct? Did the rule over this change?
You are correct that the exact combo of a Black Lotus, Channel, and Fireball is not enough to win the game. However, all you need to add to make it work is a land that produces red mana, a Mox Ruby, or a second Black Lotus.
The original text of Black Lotus, in Alpha, was
Adds 3 mana of any single color of your choice to your mana pool, then is discarded. Tapping this artifact can be played as an interrupt.
There's no reasonable way to interpret that as allowing a player to produce both green mana for Channel and red mana for Fireball.
Note that you also need that land just to have enough mana period, ignoring colors. If you start the Channel + Fireball combo with only 3 mana, you have to pay 20 life to have enough mana to deal 20 damage.
This means the widely known one-turn win consisting of Fireball, Channel and Black Lotus could not possibly work, since Black Lotus alone could not provide {R}{G}{G}. Is this actually correct?
Yes, but you can add plenty mountains to the deck.
Play mountain. Tap mountain, R
Play lotus cost 0, (R left) Use lotus (select green, GGG R)
Cast channel for GG (R G left)
Channel 19 life to mana (1R 19 G)
Profit! I mean fireball R(x) with R and 20 as x.
