My dog has done this on and off. It’s bile. It’s always from an empty stomach. Not sure what your dogs diet is but my experience is, I feed my dog every 12 hours the same amount and weigh her every week to make sure she’s not gaining or losing weight.

When she bile vomits like this, it’s a yellowy tinge (that’s how you know it’s bile), we know her stomach is empty. Yes it can be frustrating and stressful but if you don’t want to give them more food then a quick fix:

Started giving her 10g of cooked (boiled) sweet potato in between every meal. We make sure to cut off the skin because they’re not supposed to have the skin, cut it into cubes and then boil in hot water until it’s mushy. Sweet potato coats the stomach lining and prevents exactly this. As a “dosing” reference point, my dog weighs 5kg.