Hi Wilf. I considered tea to darken a few strips. It had occurred to me when I was hiking the Coastal Strip in Washington State and we were filtering our water from a stream that empties out from a WRC forest and into the Pacific Ocean. The stream ran the exact color of tea; due, I was told, to the tannin content of the WRC (see Olympic National Park water link below).

I ended up ruling it out when I realized the color could be sanded away; that doesn't mean it won't work for your application.