In my experience, something like this that is likely to be interacted with directly by the player (as opposed to some back end or editor tool) would need to be pretty customisable to fit a game's existing art direction and interface design.
To that end, I think it'd be important to be able to customise the interface, being able to change not only the interface's colours, but also to be able to replace buttons, borders, picker icons, etc. with our own images (in a similar way to how one might do it with Unity's built-in UI components).
I also feel that games generally want to curate what colours things can be to make it difficult to choose terrible colour combinations, so having it as easy as possible to choose custom colours (and optionally sort them by hue/value) would be helpful.
I think it'd also be helpful, when trying to curate this custom palette, to be able to apply colour balance changes across multiple swatches in the colour palette. (This is something developers could be doing in an external editor like Photoshop, but even those aren't really tailored to this kind of thing, relying on plugins themselves to do things better.)
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To that end, I think it'd be important to be able to customise the interface, being able to change not only the interface's colours, but also to be able to replace buttons, borders, picker icons, etc. with our own images (in a similar way to how one might do it with Unity's built-in UI components).
I also feel that games generally want to curate what colours things can be to make it difficult to choose terrible colour combinations, so having it as easy as possible to choose custom colours (and optionally sort them by hue/value) would be helpful.
I think it'd also be helpful, when trying to curate this custom palette, to be able to apply colour balance changes across multiple swatches in the colour palette. (This is something developers could be doing in an external editor like Photoshop, but even those aren't really tailored to this kind of thing, relying on plugins themselves to do things better.)