I'm planning on tiling all of the walls in my master bath. I'll be using cement backerboards for the shower and surrounds, can I use greenboard for the rest and just tile onto the greenboard directly? I've not been able to found a clear answer on this. Thanks.
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.
The answer is Yes you can tile on green board. I have even found it in tub surrounds that were done many years prior (15-20). However the reason I found it was, it needed repair work. I have never had problems with green board in non wet areas. Use an adhesive/ mastic not thinset to attach the tile to the green board.