it could be the cayenne? Are you drinking tea or coffee? Those could be the culprits, as well. Have not used clay and water, but maybe this could be a culprit?
I personally brush with baking soda and peroxide (dang, forget to stop and get peroxide on way home from work!...tomorrow :D) and my teeth look fabulous. When I paid for a bleaching my teeth became (almost) as white as they are now. BUT the one thing I will mention (which I have before, a while back) is when I was a toddler, I was given large amounts of iron, in liquid form, due to serious anemic status. By doing this, it (somehow or other, seems it should have been just first teeth, not adult...oh well) stained one of my teeth. No matter how my cleanings went and even with the bleaching, it did not become unstained. It is no longer stained since I have begun brushing the way I do now.