When concentrated HCl is added to an aqueous solution of ${\mathrm{CoCl}}_2$ , its colour changes from reddish pink to deep blue. Which complex ion gives blue colour in this reaction?

The complex ion is ${\left[\mathrm{Co}{\left({\mathrm{H}}_2\mathrm{O}\right)}_6\right]}^{2+}$.

$$\begin{gathered} {\left[\mathrm{Co}{\left({\mathrm{H}}_2\mathrm{O}\right)}_6\right]}^{2+}+4{\mathrm{Cl}}^{-}\to {\left[{\mathrm{CoCl}}_4\right]}^{2-}+6{\mathrm{H}}_2\mathrm{O} \\ \mathrm{reddish} \mathrm{pink} \mathrm{deep} \mathrm{blue} \end{gathered}$$

Here heat is a reactant in this equilibrium. This means that when heat is added i.e., the solution is being heated, the equilibrium will shift in the direction of the products. This shift in the equilibrium will then the solution blue, the colour of the ${\left({\mathrm{CoCl}}_4\right)}^{2-}$.