Avocados do tend to turn brown after you slice into them, but the addition of mustard and lemon juice will help slow that process. I've made these eggs 24 hours in advance and didn't find them to be too brown looking, but adding a garnish right before serving will help make them look even more visually appealing. (Alternatively, you could store the filling in a tightly sealed piping bag, and then pipe the filling into the eggs the day you serve, for a fresher look.)