There's a dark band of liquid on top of my secondary fermenter. I brewed an all grain kit of Cologne Kolsch, it's been in a secondary fermenter for about 10 days and there's a dark band (approx 3/4") at the top. The rest of the batch is a lighter tan color. Any explanation other than contamination? It's the first time I've brewed this style so I don't know what's normal.