I always ask my long-arm lady her opinion of what color to quilt. She has more experience than me, knows what color will work best. Once I suggested to her I wanted a bright variegated thread because the quilt was made of bright Moda Marble fabric. She agreed with me and it turned out awesome.
I have fabric put aside for a veggie jar quilt. I was thinking (when I ever get to it) of black background, but that's me. I prefer brights, the bright veggies will stand out in black. But that's me. I use a lot of black. But I do have my collection of white on white fabrics too. It really all depends on you, and how bright your veggies and fruits are.