Peggy Wenk and others have noted that tartrazine (Colour Index 19140) is used 
both as a histologic counterstain and as a food coloring. 

When used as a food coloring tartrazine is called F.D.C. Yellow 5. It is 
probably the most widely used of food dyes - look at a bunch of labels on 
junk food and you'll see it right down there polywoofsylate, polywopsylate, 
and mutilated hydroxypigeondoody. It's actually been the subject of 
considerable controversy: a few years ago any guru worth his beard and 
sandals would tell you it caused the bratty behavior of whatever normal 
little boys are called by school psychologists this week. This tinctorial 
fancy seems to have disappeared with Organic Germanium.

To my knowledge no other synthetic dyes double as food colorings and 
histologic stains. Unfortunately most of our dyes, in particular the 
triarylmethyls  (such as fast green, the rosanilines and fuchsins, and a good 
many others) are under increasing suspicion as carcinogens and probably won't 
be around much longer.

Bob Richmond
Samurai Pathologist
Knoxville TN