In some respects your eyes are correct: if you take the image into an editor and apply a strong gaussian blur to it, you can clearly see the "average" of the st

In some respects your eyes are correct: if you take the image into an editor and apply a strong gaussian blur to it, you can clearly see the "average" of the strips within each spiral arm are green or blue. This is why the illusion is more effective at a distance: your eyes have surprisingly crappy color resolution, and zooming out has the effect of blurring the colors together. (And it's also why you completely miss the real illusion -- that each solid color in the picture is really made of tiny red, green, and blue rectangles.)