Most optical illusions are cleverly placed but believable enough that once you get an explanation, you believe it. This one was so impossible, I had to pull the image into Photoshop just to double-check.
In the image below, the “blue” stripes and the “green” stripes are the exact same color, RGB 0-255-150. The pink and orange lines are the key to the illusion. The orange brings out a more brisk green than how the strips by themselves appear, while the pink stripes make the light green appear to be baby blue.
If you don’t have a headache yet from the visual, pull it into Photoshop and check it out yourself.