I have seen it a few times. Its just the two mesh counts causing the effect. Go to a higher mesh for your top color and it will go away. You can simulate it by taking two screens and putt them together. When you move them back and forth you can see how the mesh interacts with each other. Happens mostly with greens and blues. I have even seen it on a white before when the proverbial screen door 110 is used for the base screen and a 135 is used for the highlight.
