I might go with a sage green... :)

Edited to add: Oops...I didn't pay very close attention! I was looking at the outer borders with green leaves! In the red...why not an off white or a kakhi tan? It will be high contrast in some areas but if you're consistent throughout the entire quilt I think it would be lovely. I actually think green might still look nice or even navy blue in the red sections...

I would use a bright gold color.

I'd probably use that soft/lighter green in the leaves for the whole if it were mine. :)

Jan in VA

There is a taupe that would be a great neutral.

The first color that came to mind was a light creme (off-white) color. Also a thread in the gold family. The best way to know for sure is if you have scraps and can audition different thread colors by stitching a few areas. Good luck!

either the khaki green or maybe an "old" gold

Thanks, everyone! I ordered some khaki thread....I have an Aurifil color chart and it seems to blend into the green very well. Just started using the 40 wt for quilting, and I love it! should arrive today. I am a beginner FMQ'er, otherwise, I would have used the gold suggestion!

Good choice. The khaki will allow the holly leaves to remain distinct; everything else is bold enough to withstand the contrast. Have fun.