as much as that is true.. it affects mainly montis and certain colours take a greater tow from a lack of it.
but of course this is usually the last few things to look as compared to great water conditions and stable swingless parameters.
as for potassium, if theres good husbandry and a lack of it still occurrs(applies for every other element too), it could very well boils down to the saltmix. (which usually ensures you the major good stuff)