I am mostly a dressmaker--so take this with a grain of salt, but when dress fabric shows through like that, I underline it, or in this case, line it. Try laying a solid white, natural, and dark fabric under it--one at a time, of course. See if one of those stops the shadowing and looks nice. If it does, you could line the entire quilt. You'd need to attach it to the quilt--maybe stitch in the ditch would work if you want to machine quilt it. Otherwise, I'd baste the top and lining together thoroughly, then baste again to the batting and backing and then, finally, quilt it. My favorite choice would be Kona premium muslin--it's pretty light in weight--there are other choices, I'm sure. Now, let the more experienced folks weigh in.