In the change ${\left[\mathrm{Cu}{\left({\mathrm{H}}_2\mathrm{O}\right)}_6\right]}^{2+}\stackrel{\mathrm{HCl}}{\to }{\left[\mathrm{CuCl}{\left({\mathrm{H}}_2\mathrm{O}\right)}_5\right]}^{+}$ the colour changes from

One of the H₂O molecule is replaced by Cl ligend. So absorption of wavelength shifts from blue to pink