Take all the liqueurs out of the liquor cabinet. Choose a brightly colored liqueur and pour it into a clear, tall, but small glass. A champagne flute would do nicely if you don’t happen to have clear, tall, designated liqueur glasses. (Hang around flea markets long enough and you’re sure to find them if you really have your heart set on owning a few.) Then choose another color—but make sure its taste goes with the first one’s. Using the back of a spoon to break the fall of the liqueur into the glass, float the second liqueur onto the first one. Keep going until you run out of liqueurs, patience, or both.
Susan Waggoner and Robert Markel, who wrote the handsome-looking, informative, and reliably-reciped Vintage Cocktails, provide some classic pousse-café combinations:
The Fourth of July: Grenadine (red color, blackcurrant taste), Cointreau (almost clear color, orange taste), and Blue Curaçao (blue color, orange taste)
The French Tri-Color: Grenadine, Maraschino (clear color, faint cherry taste), Crème Yvette (pale purple color, violet and vanilla taste; if you like Aviations and already have Crème de Violette, use that instead).
And—sacre bleu! (et violette, et orange, et vert…) the incredible Paris Rainbow: Crème de Violette, Crème de Cassis (dark red color, currant flavor, Maraschino, Green Crème de Menthe (violently green color, mint flavor), Yellow Chartreuse (brilliant yellow color, herbal flavor), Curaçao (clear color, orange flavor), Cherry Brandy (pale red color, obvious flavor).