One of my males was doing this. It's basically bile. I've gotten my 3 cats OFF dry food completely, because it's all starches, and cats can only digest meat as "obligate carnivores." I've gradually converted them over to a raw diet, with a tiny bit of canned mixed in (for flavor/aroma). No more vomiting. My other male was howling @ 4 - 5 am, due to urinary tract crystals. A raw diet has eliminated this problem, tho he's reacting (again) to the starches in canned food, so I'm getting him off that too! Apple cider vinegar is recommended (very diluted) for urinary issues, so a few drops in their water and (his) food should rectify the problem. Also, I normally use a few drops of olive oil in my cats' food to help curb hairballs, but I've added castor oil to it in hopes it will heal the urinary tract issues in my 2 1/2 yr old and perhaps mitigate my 4 1/2 yr old (long-hair) kitty's tendency for constipation and (these days) rare hairballs. NEVER feed a cat dry food! It causes dental problems, overweight, and major health issues. My 5 yr old female lost 4 - 5 lbs in several weeks, just getting her off dry food.
