"
Lover:
Have another drink, my dark-Eyed beauty
I've got one more night left, here in town
So have another drink of green elixir
And we'll have ourselves A little mixer
Have another little swallow, little lady,
And follow me
Elphaba's mother was having an affair and Elphaba is the product of this affair. Although we don't yet know why she is green, it can be assumed it is because of the elixir.