What exactly is color? What is color?
“The property possessed by an object of producing different sensations on the eye as a result of the way the object reflects or emits light”…. You might say.
So take a white object and paint it “red”. What property of the paint now makes the object appear red?
“Pigmentation!” … you might say.
Good. Now that I’ve got you thinking in line with what I’m thinking, please help me understand what pigmentation actually is. I’m looking for an explanation that goes down to the microscopic level. What is it about the elemental structure of different pigmentation that alters the way that pigmentation (color) is seen by our eyes when applied over an object?
 A: OK the answer is not microscopic level but beyond that. It is in atomic level.


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*Metal complexes are often colored. These colors come from the d-orbitals because they are not involved in bonding. This is because they do not overlap with the s and p orbitals of the ligands. Most transitions related to colored metal complexes are either d–d transitions or charge band transfer.

*In centrosymmetric complexes, d-d transitions are forbidden by the Laporte rule. However, forbidden transitions are allowed if the center of symmetry is disrupted, resulting in a vibronic transition. The color of such complexes is much weaker than in complexes with spin-allowed transitions.

*In Metal-to-Ligand Charge Transfer, electrons can be promoted from a metal-based orbital into an empty ligand-based orbital. These are mostly likely when the metal is in a low oxidation state and the ligand is easily reduced. Ligands that are easily reduced include CO, CN- and SCN-.

*An electron may jump from a predominantly ligand orbital to a predominantly metal orbital (Ligand-to-Metal Charge Transfer or LMCT). These can most easily occur when the metal is in a high oxidation state.

*Coordination complex color results from the absorption of complimentary colors. A decrease in the complimentary color wavelength can be observed by UV-Vis spectroscopy. This decrease is correlated with the electric field of ligands, and indicates the energy gap is increasing.


These are the complex and detailed answers to the question 
A: Color is complicated. It depends on what kind of color you are talking about.
The color of a light is simply the visible part of the EM spectrum of that light.
The color you perceive an object is simple the color of the light that comes, reflected or emitted from that object.
But the real color of an object or pigment is more interesting. Put simple, it is the fraction of the (visible) EM spectrum that it reflects. Or if you prefer, it is a function that converts an input spectrum into an output spectrum. So a red object is an object that reflects some part of the redish part of the visible spectrum while absorbs the rest.
But alas, real things are not that simple. The reflected light may be different depending on the direction of the incident light. And it may be reflected differently on different directions.
Moreover, sometimes there are frequencies in the reflected light that are not in the indident one (fluorescence). And sometimes the objects have memory-like reflections (phosphorescence). And sometimes the light pass, partly through the object...
