Be careful with variegated thread, because it will likely make part of your design show up very well and part not at all. That can be an artistic addition or a problem. Picture below depicting what I mean.
A limey green would be pretty. Or pick whichever is your favorite color of the group and use that. Go a tad darker than the exact color of the fabric and it will show up pretty well. I wouldn't go lighter, because it would take away from the richness of the fabrics.
I like lighter colors like ecru but if you use them they will become prominent. I am partial to the high luster trilobal polyesters these days, so I might choose something in a light gold polyester. (Before you gasp, Leah Day and Karen McTavish both use the trilobal polyesters).
Black would work too, although it might also take over.
End of my musings ;-).
Variegated example. I considered it a design element here, liking the way it made some of my spirals disappear, but you can't predict where it will change so your butterflies might look a little funny if you use variegated. MIGHT, I say, or it could make them more interesting.
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