You could look at the GRP sheets at the home centers. Commonly used for
kitchen and bath walls where health departments are involved. Two or
three colors, usually a pebble finish (maybe you could use the back as
the finish surface). Seams and corners user plastic extrusions to join
the sheets. Easy to clean and color fast. The weak spot is probably the
plastic extrusions - don't know how they last. Come in 4x8 and 4x10 and
4x12 (I think), but you probably will only find the 4x8 at the home
center.

Rufus