"Squirrels will chew anything even if it's obviously inedible. It is possible he has been chewing on a purple ink cartridge and then groomed that colouring into his fur.
"Alternatively he may have fallen into a bucket containing a weak colour solution that has stained his fur.
http://www.telegraph.co.uk/news/newstopics/howaboutthat/3884420/Purple-squirrel-baffles-experts.html

- That's the easter bunny in disguise. 
