As of right now the game does not have a use for blue or green minerals, since the addition of sprites, and removal of the vendor who would trade crowns for minerals.
I feel a cool addition to bring back more use overall to minerals, and give some use to the currently unused ones. Bring back a vendor / machine we can use that can convert minerals into others, via color mixing, trading x amount of one, for x amount of another. Could even add a few new masks (since that seems to be a trend for recent updates) based on the sprites, and or mineral based cosmetics.
I feel this would be quite a easy addition and give more use to pet food / possibilities. Helping new players out significantly with the leveling sprite process, as well as giving end game knights more to do / earn crowns from.
Having recently maxxed out my Maskewraith I've definitely dwelled on the Minerals getting an expanded purpose again yeah. Beyond the desire I've had for a Moon-themed Battlesprite using the Moonstone mineral that's spanned more than a decade, I do feel like I can look forward to Grey Havens revisiting this exact topic when they're at full momentum.
The idea of Masks/Cosmetics and accessories themed around the minerals is a wicked idea though, since I don't recount any mineral besides Dark Matter being used frequently in terms of lore, excluding the Shard bombs of course. Otherwise, yeah, A place to trade in minerals for Crowns or other wares would be nice given they've exhausted their use for me entirely now.