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