"Shadowing" in my context, at least, and that of quilt show judges, is the showing through of a darker fabric on a lighter fabric. You may know that I'm a big proponent of layering in my quilts and generally I will try to avoid placing a light fabric over a darker fabric.
Sometimes you just have to do it though, as I did in Missouri Barn above. I wanted really white clouds where the clouds were white and I had placed them on top of a Kona fabric called "Candy" which was a perfect blue for this particular piece.
You may have noticed that a single layer of white can be really transparent and then when you get into white tone on tone fabrics they are even more so. Kona seems to have gotten a little more hefty in recent years, I know not why, but even though I was using Kona white I was still having the blue that was tucked under the edges of these clouds showing through or "shadowing".
One solution I tried was painting the backs of my white pieces with Mod Podge for fabrics. I didn't particularly like it as it made my piece a little heavy and I was concerned about having to quilt through it combined with the layers of fabric. I can get a little carried away at times and have quite a few layers loaded up. Don't get me wrong, you CAN sewing through it and if you're just doing straight line quilting that probably won't be a problem. I still wasn't getting the coverage I wanted though.
Generally I do turned edges on most of my pieces, but I combine that with raw edges on smaller pieces (knowing that on the raw edges I'll have to be more diligent in quilting them).
The solution I came up with ultimately was to use a double layer of the white, cutting the underlayer just l/8 to l/4 inch smaller. After I had turned the edges of the piece, I could lay the underlayer into the turned edges. You can also do this with raw edge pieces I think with the same result - but of course, testing first, especially as to how much smaller the underlayer needs to be.
Which all leads me to an observation.... when you're in the fabric store looking at some luscious bolt of a light colored fabric, it looks rich and saturated with color, right? Pull out a little from the bolt, insert your hand between the two folded halves of it and you might see a really different and washed out looking fabric. Once I realized that, I realized also that I could layer fabrics for lots of different effects, so don't hesitate to layer up those light colored fabrics. Once you know you're going to do it, you can just cut out two at a time.... simple.
Another possibility is using a spray of Terial Magic on the back of a fabric. I got that tip from my class with Sandra Mollon, and I've been using it frequently on fabrics that have a fraying tendency. It makes the fabric about the texture (can't think of a better descriptive word) of cardstock and yet it is easily sewn or quilted through. Again, a test with your own light fabric is required.
Sometimes it seems half my work time is testing one thing or another, but the results are worth spending the extra time and enduring the delays.
So there's my two cents on shadowing.
Every day's a treasure. Enjoy it!