The side, not used for striking the ball may have a color sheet (opposite of the other side used for striking the ball)... and on the side not used for striking the ball where you rest the fingers, you mak have cork there if you wish - many do and many J-Pen bats come that way.
You could just leave it with nothing too, the rules cover the side USED for striking the ball... but yes... there is a long tradition to make the non-striking side covered with a color sheet of the opposite color of the striking side... but that is not mandatory, unless Der_Echte wants to clean every bathroom in ITTF HQ for being wrong...
2.4.3 A side of the blade used for striking the ball shall be covered with either ordinary
pimpled rubber, with pimples outwards having a total thickness including
adhesive of not more than 2.0mm, or sandwich rubber, with pimples inwards or
outwards, having a total thickness including adhesive of not more than 4.0mm.
2.4.6 The surface of the covering material on a side of the blade, or of a side of the
blade if it is left uncovered, shall be matt, bright red on one side and black on
the other
With effect 1st October 20211, the surface of the covering material on a side of
the blade, or of a side of the blade if it is left uncovered, shall be matt, black on
one side, and of a bright colour clearly distinguishable from black and from the
colour of the ball on the other.