Crystal field theory is a way of explaining the electronic structure and thus the color of complexes. In a complex, the central metal ion has no s electrons: it has electrons in the d orbitals.

Note that the top two orbitals are aligned along the x,y, or z axes: the bottom three are aligned inbetween the axes. In a bare metal ion, all of these orbitals have the same energy. However, if you place a ligand at the far ends of each axis and slide it towards the molecule, the electrons around the ligands will interact strongly with the electrons in the two orbitals that are aligned along the axes (d_z² and d_{x² - y2}) and less strongly with the electrons in the other three orbitals.

These interactions cause the energies of electrons in these orbitals to split: this is the crystal field splitting energy, often abbreviated D. Electrons will enter the lower energy orbitals first.

We can place the first three electrons in the three orbitals according to Hund's rule: one electron in each orbital all with the same spin. For the fourth electron and beyond it gets more complicated. To place an electron in a high energy orbital requires energy, but it satisfies Hunds' rules by having 1 electron in each orbital spin aligned. If we place an electron in a lower energy orbital, it has lower energy than one in an higher energy orbital but now it has to "share" the orbital with another electron. The two possibilities are shown below for a 6 electron ion: the energy levels of the ion are shown on the left, the energy levels of the complex on the right.

In the high spin complex, electrons enter the upper levels before they pair up: D is small enough that the electrons can be in the high energy orbitals. In the low spin case D is too large to place electrons in the high energy orbitals and they end up in the lower levels paired up. The magnitude of the D splitting is given by the spectrochemical series. The magnitude of the splitting also determines the color of the complex.

Information provided by: http://learn.chem.vt.edu