I always use the same thread in the bobbin when doing FSL so I would use the variegated in the bobbin

When I use variegated thread for fsl, I use one of the colours in it , in the bobbin. HUGS Yvonne

Not a silly question!!!
Rule of Thumb for Me - I always use the same thread top and bobbin for FSL. Unless the tension is set exactly for the stitches to join right at the edge the top thread could probably be seen at the back at certain points.

Depending on the colour and amount of variation in the variegated thead, Yvonne's choice of one colour would be fine and look quite good.
To me, strong variation would indicate that one colour be used and the tension set exactly so that there isn't any of the top colour showing at the back.
I'd really like to hear from anyone who has tried setting the tension exactly like this, and also anyone who has used one of the strong colours this way. Is it possible????
AlmaG.

If you use variegated thread in the bobbin, it isn't possible to keep the colours synchronised so it can give a rather muddy appearance. I prefer to use one of the colours - which often turns out to be white bobbin thread!

Like Yvonne, when stitching FSL with variegated thread I use one of the colours, preferably the lightest. I found that using the same thread gives a less satisfactory outcome.