Let us illustrate by sodium choride `(Na^(+)Cl^(-))` crystals. Upon heating in the atmosphere of sodium (Na) vapours, sodium atoms get deposited on the surface of the crystals. The `Cl^(-)` ions from the crystal lattice leave their sites ans diffiuse into the surface. They tend to combine with sodium atoms present in the vapouus which in turns get ionised to form `Na^(+)` ions by releasing electrons. The latter are trapped by the anionic vacancies created by `Cl^(-)` ions in order to maintain the crystals electrically neutral. Now, the electrons absorb radiations corresponding to a certain colour from white light and start vibrating. They emit radiations corresponding to yellow colour. That is how, the crystals of sodium chloride develop yellow colour. These electrons are called F-centres because these are responsible for colours (In German. F = Farbe meaning colour).
