I am far from fastidious about clean my meers. Part of the reason is because years ago when finding a decent meer was just a pipe dream a few older smokers told me you have to clean the meer very well or it will start changing color. So basically the best feature can be avoided with a little bit of work, neat. I bet it's because it takes a while for the oils or whatevers to get down into the pipe. Especially since the coloring seems to be the same at the end of a smoke and then a little darker the next day. Though I don't really know why.