We prepare the mixture for the white/milk layer at the very beginning of the recipe. This is the layer that is used between each color layer. By the time you're on the 3rd or 4th layer, the mixture for this white/milk layer might have cooled off too much and look lumpy. There's an easy solution. Just microwave the mixture 10 seconds and stir. You want to get the mixture hot enough to loosen up and for the lumps to go away. But don't make it too hot (otherwise when you pour the mixture into the pan, the heat will "melt" the color layer below it). You'll end up with muddled layers instead of distinct layers.