The color of a transition metal complex depends on the specific metal ion, the ligands attached to it, and the geometry of the complex. In this case, we are given a coordination number of 4 and a d4 electron configuration.A coordination number of 4 suggests that the complex may have either a tetrahedral or square planar geometry. The d4 electron configuration means that there are four electrons in the d orbitals of the metal ion.In general, the color of a transition metal complex arises from the absorption of light, which promotes an electron from a lower energy d orbital to a higher energy d orbital. The energy difference between these orbitals, known as the crystal field splitting energy , determines the wavelength of light absorbed and, consequently, the color of the complex.For a d4 electron configuration, the color of the complex will depend on the specific metal ion, the ligands, and the geometry. In a tetrahedral complex, the crystal field splitting energy is typically smaller, and the complex may absorb lower energy longer wavelength light, resulting in a color towards the blue/violet end of the spectrum. In a square planar complex, the crystal field splitting energy is usually larger, and the complex may absorb higher energy shorter wavelength light, resulting in a color towards the yellow/red end of the spectrum.However, without more specific information about the metal ion and ligands involved, it is not possible to predict the exact color of the complex.