Roasting is a great way to deepen and intensify the inherent sweetness of red onions. Unfortunately, this process can also cause the onions to turn a deep blue-grey color as they cool. Although the off-color onions are still perfectly safe (and delicious) to eat, the result isn’t very appealing visually.
One popular theory is that red onions act as a sort of “litmus test,” with grey onions indicating a more alkaline environment. This test shows a visual side-by-side comparison of when red onions are roasted with a bit of added acidity (red wine vinegar) for balance vs. when they are not.
In this case, the results are clear: The addition of red wine vinegar prior to roasting yields brighter, more visually appealing results than those prepared without the extra acidity added.