These are spare battery covers left from new shell kits used during our repairs. If you just need the battery cover, you won't have to buy the whole shell replacement kit.
Gloss black is used in the all black shells.
Flat black (also called matte black) is used in the cobalt blue/black and crimson red/black shells.
Metallic rose is a dusty pink shiny metallic color.
Pink is baby pink, cotton candy pink, pastel pink ... not metallic.
Red is for either solid red or Mario red shells, not metallic or shiny red.