A two-person variant for Gang of Four.
Remove the rainbow 1 card, shuffle the remaining cards and deal 16 cards to each player, leaving 31 in a face-down draw deck.
Then shuffle the rainbow 1 back in the draw deck.
Play a round as normal. When over with a round, replenish your hands back to 16 from the draw deck.
Continued to play rounds and replenish until the draw deck is depleted (winner of the round gets the least cards).
Both players play out their hands until one has none left, which can occur before the draw deck is depleted.
EARLY END OF PLAY
At any point, if a player plays the rainbow one in a combination, it signals the end of replenishing from the draw deck, regardless of the number of cards left. Both players play out their hands until one has none left.
Score as normal.
We found the speed of the game was not affected, and the hands were balanced, especially when we knew certain cards were coming. A few games included playing the rainbow 1, but it was always a threat.
We found we played in ways that anticipated gaining desirable cards (e.g. waiting for the fourth 3 in a gang of four). We also played more five-card combinations, because that allowed for more drawn cards.
Another idea was to end the game on the round that included the rainbow one, but this seemed to random and sudden. It would also mean both players might have cards left in their hands, albeit one more than the other.