Aidan,
The colorless PAP pens are OK.  The colored ones could mean trouble if you
are doing immunofluorescence.  Sadly, we used the red PAP pens because it
was easier to see.  The samples were mounted with Vectashield containing
DAPI, examined and then stored at 4 c for about a month.  Guess what, the
next time I looked at the slides, they were all nice and pink and the
tissues were uniformly red.  I assume the red color dissolved in the
mounting media.  Otherwise, the colorless ones are OK and I have
fluorescence samples that I kept for months in the cold that still look
great.  Good luck

Cora
Corazon D. Bucana, Ph.D.
Dept. Cancer Biology
UT M.D.Anderson Cancer Center
1515 Holcombe Blvd, Box 173
Houston, Texas 77030
Phone: 713-792-8106
FAX 713-792-8747