The color used to make the hearts in this image is the same for all of them. The three colors you’re seeing are a dark magenta, a dusty blue, and a bright green. Your brain gets thrown off by the proximity of the magenta to the green and goes with the “assumption” of orange. The same works for the blue and green stripes, the bright green stripes look aqua next to the blue, right? The color you’re seeing in the “orange” hearts is closer to the true green.
You’ve certainly seen some examples pictures that show how your brain misinterprets color, especially since you apparently know how to use the internet. Why you might not have seen, however, is an entire collection of these illusions accompanied by “disassembled” version of the pictures to show what’s really there.
Aside from being another fun distraction from whatever you should be doing, these illusions can be useful to anyone doing design or photo work in demonstrating how the illusion works. That way, in those times when you’re trying to tone down the crazy orange thing in your image and it’s just not working at all, you’ll know the fault belongs to the relationship between green and red. Oh, and yes, I said green and red make orange. That’s how a computer monitor works. It’s basically “two reds plus one green”. Yellow is also a mixture of green and red, with the ratio being equal (or close to equal).
As you’ll see on the website, it works with greyscale too.