Can we do this? sealed with value classes

sealed interface Color {
    @JvmInline
    value class StringColor(val value: String) : Color
	@JvmInline
    value class IntColor(val value: Int): Color
}

Your code will work, but yes, autoboxing can always happen. I really like this guide for explaining when autoboxing happens https://typealias.com/guides/inline-classes-and-autoboxing/ To best avoid autoboxing you'd want something like a single Color value class, but with multiple constructors for string/int values. But what you've written works fine too, and autoboxing is unlikely to be a big problem, so I'd be more cautious about preoptimising!

But we’re not gaining anything with value class either