How many different colors are in this spiral?
Three. The blue and the green are the same color.
Sorry, no, it’s 4 colors. The center pixel is black.
The blue and green are not the same color.
He’s right. I opened up in Photoshop, they are both RGB(0,255,150).
If you zoom in on it, you can watch the colors start to look the same.
Huh! I admit defeat. And eyestrain. 
Yep. This is one of those standard illusions that show how proximity of colors and shades effect our brain’s perception. Here’s another famous one. I’ll spoil it, because it’s obvious what we’re going to be getting at, but A & B are the same shade of gray.
I suppose at some point, someone will bring up that damn dress.
I knew the answer was supposed to be 3, but I couldn’t blow the feature up large enough to see the colors become the same.
On my monitor, there are actual blue lines being interpolated between the pink and the aqua. Blowing the picture up four times in photoshop actually looks quite different to blowing it up four times on your screen.
The illusion also works if you rotate the colours
I say 4. Am I right?
Counting the 3 main colors, the black dot and the aliasing between orange and aqua and between pink and aqua, there are 6 colors total.
Aliasing is the single pixel between the 2 colors.
I think you mean anti-aliasing, but this gif doesn’t use it. Not counting the black pixel, the palette has only three colors: orange, magenta, and sea green.
I count three colors.
I checked in Irfanview which is how I figured out it had the black dot. 4 colors. If it were a jpeg or something I could see more appearing due to compression. If your computer is showing more, then your graphics viewer is doing some seriously messed up stuff.
Your right, this image doesn’t use anti aliasing, which is why I said aliasing. And if you zoom in the aliasing looks like a 1 pixel stroke around each color. There are 6 rgb color values in this image.
The only colors are:
Orange Spiral: 255, 150, 0
Magenta: 255, 0, 255
Greenish: 0, 255, 150
Center: 0, 0, 0 (x, y 256, 256)
If you see some other colors, I would ask what program you are using to display and check the values.
That’s an artefact of whatever you’re using to do the zooming (i.e. if you’re doing it in a browser, it’s not just zooming, it’s smoothing as well). If you zoom in using something that just makes it bigger without interpolation or other niceties, the edges between the 3 colours in the images are abrupt.
Its better if you just hike it up. 
This might be one situation where the colour blind have an advantage: I saw 3 right away with no ambiguity.