Add blackberries or blueberries.
You can use red and blue synthetic dyes, too. They are typically non-reactive and are regulated for chemical content. But if you use enough you will most certainly impart unwanted flavors. If you've ever tasted cheap frosting before you know what I mean, it'll have a bitter, almost vinegary flavor to it. That's the dye. I have no idea how those chemicals will age, if that's an issue for you. I'm not sure anybody produces any foodstuffs with dyes that are intended for aging. Uncharted territory, probably.
I doubt it'll react chemically with any part of the fermentation process, but if you use enough of it to produce the same sort of rich purple color that a berry will provide you may end up screwing yourself over.
I've made purple food dye out of cabbage before, but again you're going to run into the problem of low pH flavors.
My advice would be to use a berry. If you don't want the berry flavor, stick 'em in your primary and you'll get a wonderful blueish/purple color.
