Yellow compound of lead chromate gets dissolved on treatment with...

Lead chromate is yellow in color due to their absorption and reflection of specific wavelengths of light.

${PbCrO}_{4} + NaOH ( hotexcess ) \to [ Pb ( OH{)}_{4} ] {}^{- 2} + {Na}_{2}{CrO}_{4}$

The complex formed is ${\left[Pb{\left(OH\right)}_{4}\right]}^{2 -}$

which has coordination number $= 4$ and complex is dianionic due to presence of \left(- 2\right) charge.

coordination number, the number of atoms, ions, or molecules that a central atom or ion holds as its nearest neighbours in a complex or coordination compound or in a crystal.