Tiling on greenboards?
You haven't found a clear answer because it's a matter of opinion and personal standards. 

There's nothing wrong with doing so, if you're willing to accept that drywall is much less durable when moisture is involved, and has much less impact resistance than cementboard. 

It's really that simple. I've seen entire commercial kitchens done with white drywall and 4" tile. Except for the areas behind sinks and where carts tend to bump it regularly, it works out fine.