This spiral does NOT have blue and green arms

The "blue" and the "green" are actually the SAME COLOR. The RGB numbers are 0, 255, 150, which for convenience we will call bluish-green (see small inset, left, which contains a sample from the greenish arm and one from the bluish arm).

The reason the spiral arms appear different colors is that when the bluish-green is embedded in purple in looks blue, and when embedded in orange it looks green - as shown in the bottom magnified view.

There is more explanation of this fascinating optical illusion at the Discover Magazine link, and some discussion at Neatorama. Via J-Walk.