A mysterious pink stain

I was recently called by a lady who had bought a property I had redeveloped to ask for my assistance.  

She had noticed a mysterious pink stain had appeared on the cream wallpaper, my first thought was some sort of brick dust stain which I have seen before.  When I went to see for myself, the stain was near to the top of the wall in a few blotches and bright pink. Highlighter pink!

I called in my decorator who was also confused, after lots of scratching of heads we decided the only answer was to peel off the wallpaper to see what was going on.  

Underneath the paper was a small isolated damp patch which had reacted with the paper turning it this bright pink colour.  I always keep spare wallpaper for such an event so we treated the area (which had now dried) and replaced the paper. Good as new.

I have noticed on some of the spare paper that where patches had been in contact with water they had also turned pink or purple.  Clearly some sort of reaction with the vinyl paper, but definitely a new one to see such a bright stain on cream paper.  I would expect damp to show as brown.

I might be giving that brand of wallpaper a miss from now on.  Imagine spilling a drink near the wall!!