I would definately use same thread in bobbin as the top for FSL

I have done this and I used a solid color that would support the variegated. I found using variegated in the bobbin as well muddled the colors. In the end it is a personal preference. Have fun

I love using variegated thread for FSL! I use the same on top and bottom, but I think I'll give crafter's idea a try too.

I used bobbin thread in the bobbin when I used a pastel blues colored one and it looked OK, the back was just a paler color of the pastel. In a darker color variegated I might use a medium matching color.

Any time I use variegated thread for FSL projects, I pick either the lightest or darkest color in the variegated thread and match my bobbin thread accordingly. I love the finished result. It kinda makes the piece reversible. I also often use a different color top and bobbin for many FSL designs. I just like the look and either side looks great.
Hugs

depends mostly I take one that shows up on the lightest shade and the darkest shade too, like you would pick a thread for quilting
take what you like best for the color you have

Use the same thread in the bobbin. Makes for a better looking bookmark or any FSL (free standing lace) project. Suzanne

Very interesting

Look forward to seeing what you have decided to do!