The colour imparted by Co(II) compounds to glass is

Glass is an amorphous and transparent solid obtained by solidification of a mixture of silicates of different metalsOne of which is an alkali metalAddition of small amounts of transition metal salts  to the glass  mixture imparts colour to the glassFor example, addition of small amounts of Fe+3 , Fe+2 , Co+2 , Cr+3 and Mn+4 salts impartsYellow-brown , light blue , deep blue , green or violet colour respectively