Melt the butter in a medium pot over moderately high heat. When the butter is foamy, add the onions, celery and garlic and cook, stirring, until softened, about 3 minutes. Add the reserved spinach water, bring to a boil and cook for 1 minute. Add the chopped spinach, liqueur, salt and pepper and simmer, stirring occasionally, until the mixture reduces slightly, about 10 minutes. Remove from the heat, add the cracker meal and food coloring, and stir well to combine. Cool completely before using.