Amalgamate discolorations are deposits of amalgamate fillings in the dentine.

The film shows an amalgamate filling being removed, the filling breaks into smaller pieces. What you see underneath is partly a cavity and partly amalgamate discolorations.

A cavity detector (the blue compound) is used to color in the cavity, which makes it easier to see and remove just the cavity, as it is not necessary to remove the discolorations.

Generally, dentists try to remove as little as possible of the actual teeth, but whether discolorations prevent adhesion of ceramic inlays on dentine is a topic of scientific debate, although in actual practice there are no noticeable effects on adhesion.