You can technically build GU as two colors, but there are many incentives to play at least three as you draft the towns archetype
I remember my friend, Andrew, purposely buying twelve of these just so he wouldnt have to swap them between decks, and that was in the early 2000s before we had seen reprints of it in sets beyond Ice Age
For example, if you've spent three mana on spells so far this turn and you control a permanent with an ability that triggers "whenever you expend 4," that ability will trigger the next time you spend at least one mana to cast a spell this turn
As a Human, it's also not the right Kindred typing for the most obvious small, green creature deck in Elves
Make sure you have enough graveyard recursion, re-shuffle cards like Enhanced Survillance, Feldon's Cane or Finale of Revelation, or anti-mill win conditions like Laboratory Maniac as backup plans for when you surveil a bit too vigorously
However, that situation was resolved via an errata a change to the cards official rules text rather than a ban