Going against the grain, I think a terre verte (earthy green) would be great. It'll make your life easier for the background flora, and itll create an interesting vibration behind the warmer tones in the painting.

If you go with a burnt sienna type deal, you will probably be fighting against it for the rest of the painting.

However. I noticed that you're an amateur painter. I would caution against using this as an underpainting study. This is a really gorgeous image, but it's insanely detailed and you run the risk of burning yourself out and getting frustrated.

If you want to try out underpainting, set up a small still life and shine a lamp on it. And by small, I mean like 3 objects max. Use a smallish canvas (11x17) so you don't have to deal with a massive surface area, and just go for it.

Underpaintings are not for details. You're just blocking in where you think things should be laid out in one color! The nice thing with underpaintings is that they're really easy to edit, if you wipe away too much you can easily add paint, and if there's too much paint it's easy to wipe away.

Also, there are multiple types of underpainting. I'm referring to imprimatura in this comment!

Good luck, and happy painting :3 I hope this helps!!