The inside of our kettle is lined in a blue-green deposit. Our water is very hard, but what minerals could cause the coloration?
This is not caused by any mineral in the water, but an oxidation of the kettle's steel itself. Several monolayers of metal oxide are stacked onto each other, basically forming a dielectric mirror.
This might be dangerous! StackExchange is not necessarily the place to check this with.
If this is CuSO$_4$, that's actually mildly toxic. It could also be something like the German "Grünspan", a copper rust that makes a patina on copper house roofs that's got a blue-green color. It is a mixture of copper hydroxide, carbonate, acetate, and maybe others. This is all not good if this is your drinking water.
You should get that checked by professionals. However, ammonia solution in excess should produce a deep blue tetraamine complex with copper(II)-ions, you could try that (although if it doesn't work, that's by no means a safety assurance).
Have a look here for an example of Grünspan:
Limescale consists mainly of calcium carbonate but can also contain other compounds depending on water composition. These include iron hydroxides which while a white solid gains a greenish tinge in the presence of oxygen. That may explain a green limescale in your kettle.