Explanation:

• Yellow milk is commonly related to the diet that that cow is on.
• Grass is the most common diet that will turn milk (and fat) yellow, due to a compound in the grass that makes it green called Carotene.
• Carotene is a kind of plant-steriod that makes plants the vibrant colours they are, and this carries through the body of the grazer that eats these plants.
• Carotene mostly affects the colour of fat.