Choosing a material to cover wood and paint with emulsion
Depending on how fussy you are, the best thing to look like drywall is drywall. It may also be the least expensive thing that looks like drywall.

I have built a bookcase (attached to the floor and ceiling, so functionally a wall) with a drywall backing, mudded the drywall as usual and indeed the other side looks just like a wall. You have to accept that you have limited attachment points for the drywall (unless there's a terribly thick back on the shelf, which would be unusual) and that will upset folks that want it attached as if there were studs there, but this has not proven to be a problem as far as I can tell. I carefully marked where the fixed uprights and fixed horizontal shelves were, and screwed into those. 

You could probably use resilient channel if you wanted a better screwing situation, but you give up some space and add issues treating the end to cover the gap that would create behind the drywall.