What can lead to my vomit to become yellow and bitter?

Biliary emesis: Bile is typically a golden yellow to green color. It mixes with food to help digest dietary fats. When bile backs up into the stomach, it may appear in your vomitus. When persistent, it can lead to alkaline reflux and stomach irritation. Lots of common things can cause bilary emesis, but if persistent, worsening, or associated with pain, tenderness, weight loss, blood then please have it evaluated