I've not used cantaxanthin on alpestris, by feeding them on a diet of earthworms, Daphnia and bloodworms, they are already orange enough for my taste.
However, cantaxanthin has been effective at colouring-up adult Bombina, so it's not just juveniles this applies to.
Much has been written about using paprika over the years, but I'm not sure about how speculative this is. I can say for sure that cantaxanthin works, with no ill effects when used as described above.