Here are the two pieces. They look the same. Are you surprised? This observation tells us something important about color and about the brain. "Color" is actually not something "out there in the world". Instead, color is constructed by your brain, based not only on light coming from a particular place but on the entirety of the information it gets from your eyes. And what your eyes detect is not color, but wavelengths of light (see Variations in Perception and their Significance).

In the case of the spiral pattern, the "blue" and the "green" portions are sending to your eye the same combination of wavelengths and intensities of light. It is the context, or the surrounding patterns of purple and orange stripes that contributes to the perception that the two are not the same color.

Let's look at what other colors might appear given this surrounding environment of orange and purple stripes. The draggable pattern below consists of everything except the "blue" and "green" parts. That is, the image contains only the orange and purple pattern on a transparent background. Drag the pattern onto the different swatches of colors below. How does the orange and purple pattern affect the background? What two additional colors do you see in each case?

Bad Astronomy blog: The blue and the green
Contrast/Color "Illusions"
What would "reality" look like outside the brain?: Variations in Perception and their Significance
Ambiguous Figures
Seeing More Than Your Eye Does
Making Sense of the "Picture in Your Head"
Chance in Life and the World
Hofstadter's Road Sign
Illusions, ambiguous figures, and impossible figures: informed guessing and beyond
The Brain Constructing the World: Synesthesia and Perception