Crystal Field Theory says that because of d-orbital splitting caused by different ligands we have 2 different energy levels for d-orbitals.
It also says that transition metals are coloured because photons excite electrons, and when they have the crystal field splitting energy through a specific wavelength of light, the electrons to go to the higher energy d-orbital.
When electrons come back down they emit the same wavelength of light and that's the colour we see.
But then why is $\ce{[Ni(H2O)6]^2+}$ green and $\ce{[Cu(H2O)_6]^2+} $ blue? They have the same type and number of ligands.