For FMQ I match my thread color to the fabric exactly. That's because I'm not very good at it yet.
You could try a clear polyester. I haven't tried that yet in my Sweet Sixteen, but I've promised myself I will try. Has anyone tried that?

I like variegated thread too, or I'll match the thread to the backing or to the main fabric. Or use a neutral if I don't have the right color for the backing/main fabric.