discoloration on paper towel on copper pipe ?

I don't know if this is normal or not. I wrap a small piece of white paper towel to cover the pipe once then put Sea Salt water on it (mixed at 1 cup water & 1 teas. Sea Salt ) After I'm done zapping I notice discoloration where the clip hooks to the pipe. It almost looks like rust. Sometimes there is a greenish tint if I let the towel dry on the pipe. Does that sound right ? I'm not giving myself copper poisoning are anything am I ? These are just left-over regular copper plumbing pipes I got at Home Depot for a trellis I made.