RE: Gouache painting experiment on Ocean Shells is pretty amazing, Creative Art by @Theia Art (Fstheia)

Doesn't matter whether you use premium gouache. All gouaches have poor lightfastness. The medium was designed for temporary illustrations to be photographed and then thrown away, not for longevity. That's why it is so flat as well, for the photography. Watercolors and acrylics are more lightfast as they are made for paintings to last. Anyway, now you know. The rest is up to you.