I'd go with white.

It depends on the look you want for the quilt. White would make a nice pastel quilt, red would make it a little wild, and dark green would enhance each color.....Take the blocks to your LQS and do a preview of what you like!

I would use a white sashing. That will really make the freshness of the colors pop.

I think green would be best.

I vote green

I think green also, just make sure it doesn't clash with the greens in your blocks. (just look at them, squint a little and if it shouts at you...it doesn't fit.)
This is how I was taught many (many :)) years ago and it seems to help.

If you have some fabs in your stash in colors you may like, just lay out a piece of, say green, and place the squares on top with a little space between. That's how I audition sashing, borders, etc.

I have heard that green is the most neutral color there is!

Green is good. Look outside, see all the shades of greens in a flower garden. They all work!

I'd go green if I could find a solid that seemed earthy. Dark foresty type color I think. If I couldn't find that, I think a bright white would be a good option. It would give it all a springy look.