My baby had brownish spit up once at about 3 months old. I wasn't working then, and she'd had nothing but bm. I figured that she had a bit of mucus running down her throat since she'd been congested, and it only happened once or twice over a couple of days, so I didn't worry about it.
I wonder too about the cows milk possibility; *I* had been eating more dairy around then (on summer vacation near really heavenly ice cream parlor), so she might have gotten more cows milk proteins through my bm.
If you have a ped you trust, though, I'd call and ask. You'll feel better.
Happy with my DH, 2 kids, dog, fish, and frogs