I think it would be better to either pre-wash everything, or not pre-wash anything, rather than just pre-washing the red. I've heard many people say that they prefer not to pre-wash applique fabrics because the unwashed fabrics fray less. I'm a pre-washer and haven't had trouble appliqueing with prewashed fabric. Some people simply prefer the feel of the unwashed fabric. If you usually pre-wash, I see no reason why you shouldn't pre-wash the fabrics for this quilt.