It vomited yellow foam in the morning and now is vomiting clear water. What causes this?

Yellow gastric fluid vomiting has many causes, most commonly gastroenteritis such as parasitic, bacterial, foreign-body or viral types, and in older cats liver, kidney or pancreatic disease. The color hints at the location.

Yellow water usually points to small-intestine disease; white mucus to general gut disease; and red streaks to gastrointestinal bleeding or ulcers. Take the cat to the vet for cause-specific treatment without delay.