The fact we often see a void in colour/camo scheme when a block is removed indicates they would have paint applied. Same with factory schemes.
I think David's point about natural wood is true and the default model look of a "varnished coffee table top" is unlikely
Wood is very seldom brown, more often wood that has been used outdoors is greyish. Check out the handle of an old shovel which has similar use to a jack block.
Either same colour as vehicle or weathered grey is my bet.