Nissa, Worldwaker turns lands into 4/4 Elemental creatures. My question is... what color are these tokens? Are they green, the color that the land in question produces, or are they colorless since the card doesn't specify?
Normally lands are colorless (there is currently only one exception). Nissa does not assign a color to the lands she turns into elementals, so they would remain colorless.
Also worth noting, there are no tokens created. I am not exactly sure what you mean by this. What happens when you use Nissa's ability is that a land becomes a 4/4 Elemental creature, while also retaining its previous properties as a land (tapping for mana, having the "Land" type, etc).
202.2. An object is the color or colors of the mana symbols in its mana cost, regardless of the color of its frame.
202.2b Objects with no colored mana symbols in their mana costs are colorless.
202.2e An object may have a color indicator printed to the left of the type line. That object is each color denoted by that color indicator. (See rule 204.)
202.2f Effects may change an object’s color, give a color to a colorless object, or make a colored object become colorless; see rule 105.3.
The land-become-creature has no mana symbols in its mana cost, it has no color indicator, and no continuous effect sets its color, so it is colorless. Just like it was before becoming a creature.
As previously mentioned, there are no token involved. A token is a created object that isn't represented by a card. No object is created, some characteristic of the cards have simply changed. Just like when Giant Growth is used.