There are actually no yellow pixels in this optical illusion. See below for proof.
Although the color may seem to change as it passes from the image to the white background, in fact the color is the same in both circles and the bar between them.
This is an example of a color constancy illusion, in which our brains over-correct to account for what may look like a blueish shadow and tell us what color the bananas “should” be.
If you enjoy optical illusions like this one, you will want to check out the work of Akiyoshi Kitaoka, who is a master of color illusions and other types as well. Many of Kitaoka’s best illusions — and hundreds of works by other artists as well — are collected in the book Optical Illusions: The Science of Visual Perception, by Al Seckel. This is one of the best collections of optical illusions I’ve seen. It includes some classic illusions and many that I had never seen before and that aren’t available online. To see a preview of what’s inside this book, click on the image to the right: Amazon makes several images from the book available, and you can also use the Look inside feature and Surprise Me to see other pages.