Most taste experiences are an amalgam of taste sensation on the tongue and smell sensations in the nose, so to be completely correct this isn’t purely triggered by a taste, smell is certainly an element, but in plain-language terms, the trigger is a novel taste or flavour.
This only happens during the unusual situation in which I am at a public swimming pool or some other place where I have the smell of chlorine in my nose and I am also drinking iced coffee, and there is some kind of chemical reaction between the chlorine and the coffee in my mouth/nose resulting in a peculiar smell/taste that is somewhat like a floral or perfumey smell. It is a black-coloured smell/taste. Sometimes the image of a black-coloured flower flashes into my mind, shaped something like a simple lily. Upon reflection I believe that it is the surprise or novelty of the modification of the usual flavour of iced coffee that is the synaesthesia trigger or inducer. Often as an afterthought after this experience I realise that the normal taste of iced coffee is a brown-coloured taste, but I never notice this as it is such an ordinary thing that it kind of stays below the level of consciousness.