The reason why we are perceiving one color as two different colors is because of the other colors surrounding the stripes.

Each eye has six to seven million cones, which are concentrated in a central yellow spot known as the macula. These cones measure color in different wavelengths, where some of which overlap. Our brain then compares by measuring the differences in wavelengths between the colors. When certain colors are combined, the brain can’t process the data from the cones correctly and we get confused.