The complex in ${\left[\mathrm{Co}{\left({\mathrm{CO}}_3\right)}_3\right]}^{3-}$ has one absorption band in the visible region of the spectrum at 640 nm. From this information 

The light absorbed is in the orange region (640 nm) of the spectrum therefore, the light transmitted (the colour of the solution) is blue. From the wave length of light absorbed confirms that $∆$₀ is small. So ${\mathrm{CO}}_3^{2-}$ is weak ligand. Hence the complex is high spin and paramagnetic with 4 unpaired electrons.