gORCUS:
Brown stuff is an epoxy putty just like green stuff, but with slightly different qualities.
It sets up harder (but is more brittle)
Holds detail better (but is a little softer/harder to work)
It sets up faster, but not so much as I was lead to believe.
It can be freely mixed with green stuff to get a nice compromise (and miiliput, but I’ve yet to use milliput myself).
I live in the US, I get mine from reaper and gale force nine.