Mike Faithfull wrote:

I saw some MDF yesterday that looked 'normal' until I noticed the cut edge -
only the two flat surfaces looked like MDF, the core of the material was a
greenish grey colour. It seemed to be the same density and texture, but
.......

Is this something special? The house owner didn't have a clue about it, and
the builder doing the job wasn't around to ask ....

I have heard that there is a waterproof variant of MDF although never knowingly
seen it. Maybe this is what you saw?

Bob