I don't think they've done much with histologists, but Earth Guild, in 
Asheville NC, offers natural dyes for fiber artists. My daughter Miranda - 
who's a sophomore at Harvard and a weaver and dyer, has had summer jobs with 
Earth Guild for some time and knows them well.

http://www.earthguild.com/products/dyes/dyenat.htm

Bob Richmond
Samurai Pathologist
Knoxville TN just over the mountains