I've had good luck pouring chocolate martinis (vodka, light creme de cacao)
with a little (1/4oz - 1/2oz) dark creme de cacao poured slowly and gently in
the middle of the glass so that it settles at the bottom of the
martini/cocktail glass. I believe this works because the dark liqueur is more
viscous than the vodka/liqueur mix. If you were to use something as thick as
grenadine your results - in terms of separation of the two liquids - would be
even more pronounced.
I think it would work well.
If it were red, it may likely have been grenadine, but could have been any
thick/viscous liqueur.