Neutron stars pack about the mass of our Sun inside a sphere just 10 kilometres or so across. The crush is so great that electrons and protons, the electrically charged constituents of atoms, merge to form neutrons. The core is denser still. The quarks in protons and neutrons come in two flavours, denoted 'up' and 'down'. A third type appears in the plasma, denoted 'strange'.
Quarks also come in three 'colours', terms for a property a little like electrical charge.
This CFL phase, the researchers now show, must contain equal numbers of up, down and strange quarks, rather than mainly ups and downs as had been assumed previously. If there are as many strange quarks as up and down quarks, the plasma is electrically neutral overall, and so acquires no 'free' electrons. It then becomes an insulator, like diamond, and thence transparent to light. A blob of quark matter dominated by up and down quarks would have a positive electrical charge, and so would accumulate negatively charged electrons to balance the charge. These would make it metal-like and opaque.