Each of the following forms a coloured aqueous solution except:

$\mathrm{Zn}{\left({\mathrm{NO}}_3\right)}_2$ (zinc nitrate) most often found as $\mathrm{Zn}{\left({\mathrm{NO}}_3\right)}_2\cdot 6{\mathrm{H}}_2\mathrm{O}$ is white, crystalline salt.

Zn has filled shells. hence it has no free electrons. Since the electron has additional stable orbitals, visible light cannot excite it, giving its compounds their colourless nature.

Hence zinc nitrate does not form a coloured aqueous solution.