Yellow color of NaCl
An F-center (or any color center) is a point defect in the material that generates localized electronic states. When these states are in the band gap, they will result in strong peaks in optical absorption. An F-center in an alkali-halide material is an electron bound to a negative ion vacancy.  A brief overview may be found in Ashcroft and Mermin's Solid State Physics, Chapter 30.