Green Stuff is a two part epoxy putty that has an elastic property making it suitable for organic sculpting. It stays quite flexible after curing. This also makes carving, drilling, sanding or scraping extremely difficult.
Brown Stuff is an epoxy putty the has a more traditional clay-like feel to it making it suitable for non-organic sculpting, like weapons and armor. This makes it very suitable for carving, drilling, sanding or scraping. This also makes it brittle and very susceptible to breaking.
Taken from
http://www.patrickkeith.com/2010/08/sculpting-tools-and-putty.html