I still love just white fluffy towels.

I would suggest white (There's nothing nicer than using a fluffy white towel). I love the colour of the mat and not matching the towels means the mat will continue to stand out.

I would use more color. I would try monochrome green scheme. I would also hunt down towels have a green/white pattern just for texture.

White fluffy towels are a safe color. For more interest, fluffy light to medium peachy pink 'or' orchid are gorgeous colors with this moss green rug, if you are looking for more color. Also, bring in a bouquet of floral arrangement with both or all colors to tie it into the scheme.

You can't go wrong with a great quality white set. If you do want to go in a different direction, I would venture across the colour wheel into the reds. I would keep it rusty, to compliment the quality of this green. If you'd like to keep it neutral, a rich chocolate or a tan would work as well.

I'd go with white, too.