Black soap? Black soap? That’s just silly! where the hell’d you use that? And did it clean your hands? I’m intrigued!

Pink has been engrained in our society as the color to make things clean, in particular by using soap. If soap were to, say, be black do you think you would use it to clean. Of course you wouldn’t. Just like with the margarine above where people were more likely to enjoy the taste when it’s yellow, so are people more likely to use pink soap, and attempt to use this pink soap to clean more.

Cleanliness is in part a matter of perception. And soap being pink makes people believe it to be cleaner. Believing it to be cleaner people use it more, thus things become cleaner. It’s a cycle, and it works. Trust me.

“The key to slowing margarine sales (and protecting the established dairy industries), however, emerged as restricting its color. Margarine naturally appears white or almost white: by forbidding the addition of artificial colouring-agents, legislators found that they could keep margarine off kitchen tables.”
Wiki Entry

This quote in particular has relevance to what Dave has said above: “In several [USA] states, the legislature enacted laws to force margarine manufacturers to add pink colorings to make the product look unpalatable…”