I finished stitching the sky behind Highland lighthouse (left) using the needle-blending technique. The same four values of blue--DMC floss #3755, 3325, 3841 and 775--that I used for the Nobska lighthouse sky (right) are at work for Highland. The spacing of the needle-blends is also the same: 12 rows down from the top, followed by eight rows of consecutive blends down to the horizon. You'd think the two skies would be identical, but they're not. Why?
When you combine three plies of one value and one ply of another, and let them play out as you basketweave, one of those values will predominate over the other. This "luck of the draw" produces a slightly tweedy effect, which will change every time you stitch. The only constant you'll see is when you're stitching with a full four plies of any value.