Miltonium...
Cows don't any crops on farmville.
You get strawberry milk from the pink cows, Milktonium from the green cows, chocolate milk from the brown and regular white milk from the brown and white cows.
Farmville does that to make things more exciting. They don`t want you to get board, or have bought everything.
Yes, at certain times of the year green cows are available in order to celebrate St. Patricks's day. There are some on the market right now, the are called the "Kelly Green Cow." Apart from the game Farmville though, in real life cows are NOT green. They are anything but green, purple, vibrant yellow, true blue and pink.
you can have as many cows you can fit on your farm
Adopt them from the News Feed
you could put all kinds of cows in the dairy farm and still get the proper money count for each . hick i got some green cows in my barn.
You buy them from the market and fill them with cows.
They do although it takes time
Green cows were a temporary item that came along with other alien-associated items available for purchase in Farmville. Temporary items change with the seasons. For example, jack-o-lanterns and tombstones were available during Halloween, and turkeys and feast tables were were available for Thanksgiving.
To harvest cow on farmville you need to click it. Your character will then go over to the cow and harvest it.
You can get cows on Farmville when your Farmville friends gift them to you or by adopting them when they wander across a neighbors farm and that neighbor posts a notification about it allowing you to adopt the lost cow.