There's nothing wrong with the color beige (or tan, kaki, and whatever other names you prefer). It is about as inoffensive as one could want, and it goes with many other colors. Having a wall in your home painted in beige renders the wall almost as invisible as white, freeing you up to decorate as you want. Although I am not sure I've ever met anyone who claimed that their favorite color was beige, plenty of people seem to like it enough when it comes to their interior paint, car upholstery, and all sorts of other things. Thus, while beige may not inspire much passion, it works well for many. There is nothing wrong with beige, and there is nothing wrong with people who like beige.
Beige Atheism
Atheism, at least if we are talking about the dictionary sort of atheism (i.e., atheism means nothing more than a lack of belief in gods), is rather beige. I suppose we can't call it inoffensive since there are still plenty of religious believers who seem to have real difficulty merely acknowledging the existence of atheists, but I don't want to dwell on them here. But many of them also seem to have real difficulty comprehending the continued existence of monkeys, so I think we can set them aside as they muddle about their "magic" world and get back to the atheists. Beige atheism is great for many atheists. It has everything we want and nothing we don't. Much like the color, it goes with almost anything. There's nothing wrong with beige atheism, and there's nothing wrong with people who like beige atheism.
Of course, not everybody likes the color beige. Some find it boring or otherwise unappealing. I am one of them. I once passed on buying a white car even though I really wanted this particular white car just because it had a beige interior. And yes, I once persuaded my wife-at-the-time to paint an entire room in the house we were living in at the time bright red instead of its previous beige. I remember standing there when the first wall was finished and wondering if we weren't a bit crazy. "Well, it is definitely red!" It was, and the expressions on the faces of most of our guests suggested they found it shocking, but I loved it. It is okay not to like beige, and yes, it is okay not to like beige atheism too.
People who don't like beige atheism feel that way for many different reasons. For some, it seems incomplete because they are looking for something more than just a lack of belief in gods. Something like secular humanism might appeal to some of them more than plain old dictionary atheism. Others might be content with beige atheism but recognize that this is only one small part of who they are and what they are interested in. They might still define themselves as atheists using the narrow definition but spend more time pursuing political activism that has nothing to do with atheism. Still others find beige atheism so boring, unappealing, or aesthetically uninspiring that they seek an alternative that, while still atheistic, bears little resemblance to beige atheism: Satanism.
Satanism as an Alternative to Beige Atheism
As someone who is not a Satanist, I recognize that I am limited in my ability to convey why those atheists who are Satanists were drawn to Satanism or what they find appealing about it. The best I can do is explain what I have always found appealing about it and what I have heard from several self-identified Satanists about what attracted them to it. Interestingly, there is a great deal of overlap here. What I have always found appealing about Satanism is the dark aesthetic (i.e., the imagery, the art, the music) and the embrace of the strong rebellious adversary. As a horror fan and a metalhead, I've been drawn to the dark aesthetic and Satanic themes longer than I've been an atheist. That didn't change when I recognized that I no longer believed in gods (or a literal Satan). I've also admired the adoption of a position of unapologetic nonconformist by embracing the imagery of the Christian boogeyman. Kind of like freethought on steroids with devil horns!
While I have heard many Satanists mention the same things when they describe how they found Satanism and/or what they like about it, it is true that some mention other aspects of Satanism that I find less appealing (e.g., ritual, magic, libertarianism, social Darwinism). That they find some things appealing that I don't find appealing goes a long way toward explaining why I am not a Satanist. Still, I'd be lying if I said it held no appeal. I have little trouble understanding why other atheists would embrace Satanism, and I can admit that there are times when I wish I could do so too. For now, I'll just remain a beige atheist who fantasizes about decorating his home to resemble the set of Suspiria and recognizes that I don't need to be a Satanist to do so.