One of the Techniques Stampin' Up! showcased at Leadership earlier this year, was stamping with the reverse of Photopolymer stamps.
This is a really fun technique. You just flip your Photopolymer stamp over and stick the normal stamping side to your acrylic block - leaving the smooth back side to be inked up.
I used Old Olive ink for this card and stamped it 'off' before stamping it onto the card. I then cleaned the stamp and flipped it over again, so I could stamp with the 'normal' side and used the full strength ink for stamping the foliage on the tree.