What does it mean when pickled onions get yellow spots?

The yellow spots that you see on your pickled onions are harmless and are called "browning spots." They are caused by the oxidation of the polyphenols, natural compounds found in onions, when the onions are exposed to light. While the spots may not be aesthetically pleasing, they do not affect the taste or safety of the onions. However, if the onions develop any strange odors, colors, or taste, it's best to discard them to avoid any potential food safety risks.