What is the meaning of "Fifty Shades of Grey"?

This was a topic of discussion in my book club last year. I was referred to the following article:

So what is the title of Fifty Shades of Grey referring to? There are a few options. On the surface, Fifty Shades of Grey means roughly what you think it means: Christian Grey has a lot of facets of his personality. He can go from gentleman one minute to S&M manic the next. The book is showcasing all those shades of Grey's character.

But the title is also a play on words. The phrase "shades of gray" usually refers to a situation that is not clear, particularly with regard to whether or not something is categorically evil. When doubt comes into play, things are neither black, nor white, but are in a gray area. Of course, Christian's last name allowed the author to title the book the very clever Fifty Shades of Grey with it's unique spelling of his last name.

As for the number 50, there are a lot of speculations as to why the author chose that number. Interestingly, data has found that the human eye can only see 32 shades of gray. So perhaps by choosing a number higher than that, the author is telling readers that you can't see all of the facets of Grey's character, and therefore you cannot judge him if you don't have the full picture. It reinforces that theme of "shades of gray" confusion.

A shade is a demon. Fifty Shades means fifty demons. Throughout the trilogy there are frequent references to Christian Grey's demons; those are his shades. (NB Pay close attention to the meaning of the names throughout the trilogy.) The fifty demons comes from a Buddhist book titled 50 demons.