JUST FOR MEN, the company that makes hair color for men, has created a product (that they're offering for free if you call or visit their website) which allows you to temporarily color your hair to see how would look if you were to use their product. It darkens your hair and takes out the gray – but washes out easily with a single shampooing.
This is great marketing. They have made it easier and cheaper for customers to try their product. There are no gimmicks, just a real attempt to prove that they have a good product.