Beautiful job! I would use brown in the bobbin to match the back. On the top, I would use a varigated thread, with either the green or the gold. There might even be one that would have some of all 3 colors in it. The reason for varigated is in case some of the thread pulled through from bottom to top.
Or if you wanted to do stitch in the ditch, match the top thread to the block, and use brown for the bobbin.