Red onions got dark on top?

Anyone know what's going on here? This jar of red onions and peppercorn finished about a week ago so I took out the weight and threw it in the fridge a few days ago.

Just saw it while cleaning and the top is like a darkish brown color, what's the deal? It doesn't smell off or anything, it still smells like it did when it finished.

They also never turned that deep red color i see a lot of other pics on here showing with red onions, just a light pink but they never smelled or tasted off so I assumed all was good. I only tasted them once but now afraid to try em.

Any ideas?