I just made my first cutting board from a chunk of black walnut. It looked great after sanding, but as I applied hemp oil on one side, it turned greenish. So I decided to make my own oil by mixing coconut oil with beeswax and applied it to the other side. It promptly turned the same green colour. The only clue I can give you is that while the wood feels very dry and was easy to work with, it was only cut about 3 months ago. Could the green be a result of the wood still being damp?