Ms. Brown was an M&M introduced in early 2012 in a Super Bowl XLVI commercial. That's how we found out the face of the brown M&M.
Ms. Brown appeared in more commercials to promote finding an "all brown" bag of M&Ms, as well as 'The Apprentice'.
Ms. Brown only appears on the bag for mint flavored M&Ms, which are rare. She appears alongside Ms. Green on the wrappers.