Why does putting a lid over boiling red cabbage help it keep its red color?

Well, honey, the reason you put a lid on that boiling red cabbage is to trap in all the acidic goodness. The lid helps to keep the pH stable, preventing the red pigment from breaking down and turning all funky colors. So, if you want your cabbage to stay red and fabulous, just slap a lid on it and let it do its thing.