Explanation of the purple cow can be found on this page.
As a french I like this part:
While driving through France a few years ago, my family and I were enchanted by the hundreds of storybook cows grazing in lovely pastures right next to the road.The idea behind this book is:
The world is full of boring stuff -- brown cows -- which is why so few people pay attention. Remarkable marketing is the art of building things worth noticing right into your product or service.