Can you tell?
There actually is a different one, and most Westerners can't tell which green is slightly off. Even though I know which one is different, I'm having a hard time noticing it. You can click here if you want to see the RBG values to see which one it is.
There is a tribe of of people in northern Namibia that can easily point out which green is different. That is because they have different words for those shades of green. They do have a hard time telling blue and a certain type of green apart because those are the same words. Some researchers believe this is because learning the names of colors and the perceptions of colors are linked.
This is the video that is associated with this image. It's kind of long, but really interesting. I would watch at 2:10 to get the explanation of the Himba color words.
Oh yeah, the green square they point out in the video as the different one is wrong. Or maybe it is a different chart. But just to make sure that the chart above is correct, I uploaded the image in Photoshop and double-checked the RGB values with the eyedropper tool and the linked image is right.
Updated: Sorry, I thought I included a link to the original post that I saw on Boing Boing. Here it is! It's really interesting. Man, I love that blog so much.