An animal with very bad mastitis (udder infection) could have a tinge of blood in the milk.
I'm guessing this is a joke and the answer is a pun (rosie the cow), or else has to do with the rabbit on the package of Strawberry Quik powdered drink mix.
I remember someone telling me he knew a farmer who was new to the game bought a job lot of cheap carrots and fed them to his cows which produced pink milk. I also found that shreded carrot given to hens produces a wonderful golden yellow yolk.