The black and tan idea might work with a dash of grenedine poured into the
glass, not the shot. Grenedine is heavy and usually sinks to the bottom so
you would have a red base, gold Bass, black Guinness. Just an idea, not sure
how it would taste though. Have you maybe thought about a Black Velvet
(Guinness and hard cider instead of Bass)with Grenedine instead? It might
lend itself better to the sweetness of the grenedine.
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"Beer is proof that God loves us and wants us to be happy." -Benjamin Franklin