Purple Cow is one of the best marketing books EVER.
There are three reasons the book sold well, and continues to sell briskly today:
1. The idea — To be a successful brand, you must be remarkable.
2. Seth's writing style — He's direct, helpful and illustrates his ideas well.
3. His metaphorical imagery sings. This is very very important. We love the color purple (think Prince). We like cows (milk, cheese and steak). Had he titled the book Fuschia Ferret, we wouldn't be as excited about the book — even with points #1 and #2. Why? A purple cow is more likeable than a fuschia ferret. The color fuschia is a blah pink and ferrets, well, they are mean little creatures. I've been bit by a ferret twice and never by a cow.
Here's the point: A metaphor needs to create an image that accentuates its point. Metaphors are not technically correct. They are, in the mind's eye, visually powerful.
When a product or service is a purple cow, it creates surprise and delight which makes it stand out and drive preference. That's a big idea, conveyed with a smoking metaphor. The choice of color and animal are important, as much as the concept behind it.