$50 says you can't correctly guess the color of the two circles in the picture.
Unless you guessed that they're the same color, pay up! Look at the 2nd picture if you don't believe me.
What you just experienced is a powerful chromatic illusion known as the Munker Illusion. It demonstrates the way our brains interpret color. The trick is in the color of the foreground (the stripes cutting through the circle) and the color of the circle. It is primarily a function of the color of the foreground. Meaning, the color you see will be a combo of the foreground and the circle, but it's most affected by the foreground color.
Variations of the illusion have sparked viral debates that set brother against brother and friend against friend. I.E. that stupid picture of the gold and white dress (it's gold and white!!).
As with many illusions, the Munker Illusion has been used by philosophers to show that we can't trust our senses. That any observation which is dependent on sensory inputs is inherently unreliable. Do what you will with this information, but I find it pretty compelling. The fact that you can't snap out of the Munker Illusion is what blows my mind. It's not like the bunny or duck picture, where at some point you can see both. No matter how long you look at the Munker Illusion, your brain will never get it right.