This is what I have read in " The New Complete Great Pyrenees"
book by Paul Strang.
"When all-white dogs are mated to all-white bitches, there is a
grave danger that pigmentation will breakdown. If this is carried
for several generations, missing pigment is almost a certainty. In
addition, one approaches the problem of albinism,including deafness
and impaired vision. Accordingly, only the most knowledgeable
breeder who is familiar with the markings and pigmentation of all
the dogs on both sides of the pedigree for at least three
generations will attempt to breed two pure-white Pyrenees"
Lisa