I usually go with my gut :) I look at the colors and decide like I did a Key Lime Pie and even though the pattern was supposed to be a totally different color I chose a reallly pretty green color and it turned out beautiful :) But I also agree with the above that antique white or off white is always a good choice :) That or maybe a very light tan/brown :) Good luck!
If you havent bought the fabric you can always ask what floss is the closes color match to the fabric.
I usually use white, off white or Raw Linen. I have also used a grey blue but that is it.