Belfrybat, I'm loving those fabric Cs. I'm obsessed with the one that looks like watercolor flowers, it's gorgeous!
joe'smom, I know you went with different fabrics than the ones in your picture but I 100% agree with your thinking:
I'm thinking maybe something more subdued would serve to better showcase the C fabrics than something more distinctive like the pasture scene
Given the descriptions she uses when making fabric recommendations for A & B, I'm thinking those are going to be used both for a place to "rest the eye," so to speak, from all the scrappy fabrics & to provide contrast for those C fabrics. So I figure something simple and subdued but with high contrast to our Cs is going to have the highest chance of success. I do think your blue and cream were good possibilities though, and I'm eager to see the new potential A & B.
Rebaquilts, color wheel opposites are a great choice too!