Don't panic! it's supposed to turn pink.

So, you've just finished scraping off the lichen and scrubbing the gravestone with water.  Now it's time to apply D/2 Biological Solution to kill the lichen and whiten the stone.  You spritz some on the damp gravestone and...oh no!  Why is the gravestone turning pink?!  Don't worry--sometimes stone will turn a subtle color of pinkish-orange after applying D/2 Biological Solution.  The manufacturers of D/2 explained it to me that the color change is the result of the biological organisms reacting to the D/2.  This temporary discoloration can take on shades of orange, pink, or gray.  Not all stones react this way but some will.  When it does, it will generally take 24-48 hours before the discoloration disappears.  So, the only downside is that you may have to come back after a few days to take some good "after cleaning" pictures.