Sounds like bile. Unusual there's a "significant amount" if the stomach is empty. Have you had any hairballs coughed up before? Hopefully it's a one-time event, but if it happens repeatedly, I suggest going to the vet. The worst-case scenario is an intestinal obstruction.
This is exactly what just happened to one of my female cats. She has not had any surgery recently so no post-anesthetic response. I'm concerned because she has been hiding under the bed for the past 2 days except to come out and vomit.
And the consistency of her vomit is very thin, almost foamy. No food particles, no foreign items, no hair. I think I'm going to have to take her to the vet. Any ideas? Has this happened to anyone else?