Amethyst used as a noun to denote a colour is not simply a purple colour but actually denotes the colour of amethyst. Thus used as an adjective it is used to describe something which has this colour. Something either has an amethyst colour or doesn't have an amethyst colour. Thus used as an adjective it is ungradable. As a consequence it does not have a comparative or superlative form. Even if amethyst was gradable it is multisyllabic and not a common well-known adjective. As such using the suffixes "-er" and "-est" can only lead to confusion. As a consequence the appropriate comparative and superlative forms would be "more amethyst" and "most amethyst" respectively. Because it is not gradable the only sort of comparison that could be valid would be to describe a colour as "more like an amethyst colour than some other named colour".