If he says yogurt is white, I’ll say it’s black (EN) – This idiom means going agains what you say.
It’s very nutritious, for example, yogurt has been found to reduce the risk of heart disease and osteoporosis*, as well as aid in weight management. It contains a lot of calcium, a mineral necessary for healthy teeth and bones. It’s also high in B vitamins and Proteins and It May Strengthen Your Immune System.
* skeletal disease, decrease in bone mass density