I vote a medium to dark green, but not TOO dark.
I would take the squares to a quilt shop and pick various shades of green and see what works best.
I personally like white, but I think in this case, white would wash them all out. That's just my opinion.
I guess, after thinking about white again, though, it would depend upon how WIDE you are going to make the sashing. If you made it very wide, I think white would wash the colors out.
If you made it a THIN sashing, then I think white would be excellent.
Just remember - there are NO quilt police. This is YOUR quilt, so you should pick what YOU like.
To be honest, when I first started quilting, some 30 years ago, I ended up buying fabric at my local fabric shops, because they were the choice of the fabric shop owner, not MY choice, and a lot of those fabrics are still in my sewing room... unused...
Brenda