(b) In ${\left[\mathit{N}\mathit{i}{\left({\mathit{H}}_2\mathit{O}\right)}_6\right]}^{2+}\mathit{N}\mathit{i}$ has configuration of $3{\mathit{d}}^8$.
The two unpaired electrons do not pair up in presence of weak ${\mathit{H}}_2\mathit{O}$ ligand, but in $\left[\mathit{N}\mathit{i}{\left(\mathit{C}\mathit{O}\right)}_4\right]$ due to presence of strong $\mathit{C}\mathit{O}$ ligand, it pairs up. As there is unpaired electron in ${\left[\mathit{N}\mathit{i}{\left({\mathit{H}}_2\mathit{O}\right)}_6\right]}^{2+}$ it is coloured and $\left[\mathit{N}\mathit{i}{\left(\mathit{C}\mathit{O}\right)}_4\right]$ is colourless, due to absence of unpaired electrons.