Eucidaris metularia is a common small urchin usually found under rocks or in rubble on lagoon or seaward reefs. Assuming these really are all the same species, it ranges considerably in color, as can be seen in the numerous photos below.
Side view.
Underside and mouth of previous specimen.
Some juveniles.
Another underside and mouth view.
Smaller one on top of larger one.
Created 15 June 2018