Which of the following sulphides is yellow?

CdS: Yellow color
ZnS: White color when pure and black if impure
PbS and NiS: Black color.

As the size of the cation increases, upon moving down in the periodic table, the overlap between the orbitals decreases and the band gap shifts to lower energy. Since, the band gap decreases from ZnS to HgS, the colour of ZnS is white, CdS is yellow and HgS is black.