Holmium is a really colorful elements when it’s an alloy or oxide, being able to readily change color under certain lights. It is mostly used in optical products. This is pretty much the reason I paired it with Sea Slugs that have similar color changing attributes.
Erbium is also quite colorful in an alloy. Again mostly used in optical cables and fiber optic signal enhancing. It forms often pink alloys, especially Erbium Chloride, which is intensely pink. This was the reason to pair it with the Flamingo.