Does an empty palette need its own case?

Maybe [Color] already gets you there with no third case. Nothing is the fresh edit, Just [] is the palette someone emptied on purpose, and there is no second spelling of empty to rule out.

Save is the easiest place to start, I think. Fresh edit, nothing entered, Save stays disabled. After Clear, saving an empty palette is allowed. Add blue, remove blue, and it's still allowed, because the user already made that choice.

Once that's settled, construction gets one rule. An empty list becomes Empty, a nonempty one becomes Colors, and the editing code only builds state through that one function. Colors [] stops appearing on the normal path, and you didn't need a whole nonempty-collection type to get there.

Palettes loaded from outside the editor come later. Check those before they turn into application state. The case names won't do that for you.

That Colors [] gap can be closed at construction. In Java, sealed interface Palette permits NotEntered, Empty, Colors, with NotEntered and Empty as records with no components, and record Colors(List<Color> values) whose compact constructor throws on an empty list and stores List.copyOf(values).

Removing the last color then has to produce Empty, because Colors refuses to exist empty. A switch over Palette with record patterns is checked for exhaustiveness too, so a fourth state added later breaks every exhaustive switch that lacks it. The list plus flag version gets none of that.

Just [] is still the hole oskar91 pointed at, though. In Elm I'd write type Palette = NotEntered | Empty | Colors Color (List Color), one required color plus the rest. A palette with colors can't be empty by construction, and if a case forgets the Empty branch, the compiler lists the missing pattern by name. Honestly that error message is half the reason I like the encoding.