This mod is very small, and quite simple to understand.

 

Randomly across the world, purple, blue, yellow or even black cows - and many more, will spawn.

 

These cows give random liquids when 'milked'. There are water fluid cows, lava fluid cows, molten copper fluid cows, etc..

 

And this works great with tinkers' construct!

 

To milk the cows, all you have to do is take a bucket and right-click them - just like in vanilla minecraft when trying to get milk.