Hi D'Arcy. Here's what I typically do in these situations. Take a piece of the scrap wood left over from the construction, and about every inch or so score a line across the piece of scrap (either with a saw or knife). What you want to do is create a "test" strip of wood with several sections for different test conditions. Now take a rag (actually several rags) and stain the first area with a natural stain, the next with another color stain, and so on. Then towards the end of the test strip, do some experiments... apply a coat of natural, let it dry, apply one of the other colors over top, and so on... just use your imagination and the colors you may have on hand... see what happens and what you like.
My first impression from what you want to accomplish, is to apply a "natural" stain, and follow up with something darker (say a mahogany, walnut -left on a very short time-, or a cherry). I really think the key is going to be in your experiments with the test strip... also keep in mind the the poly/varnish if oil based will tend to add a yellowing effect to whatever finish you apply.
Good luck, and let me know if I can assist further!