It depends on the quilt what seems to fit.
As a rule I do tend to gravitate towards mottled/marbled 'solids' rather than true solids. Sometimes I will also use a tone on tone print.
The exception so far being when I'm doing a really bright print that begs for equally bright solids to go with it...like the 'girl bugs' fabric with the bright lime background and bright pink and yellow in the print. I used a slightly darker bright lime and a bright yellow solid on that one. Wished I had added a bright pink piping, but I was already done when I thought of that!