Yellow color of NaCl My book says:


  $\ce{NaCl}$ shows metal excess defect, which results into the formation of an F-center in the crystal. This F-center is responsible for the yellow color of $\ce{NaCl}$.


How does an F-center provide color to the solid?
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.