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How can light absorbed by molecular orbitals affect the colour of a compound?

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Two diagrams show the three-dimensional structure of a methane molecule: panel A uses a ball-and-stick model with a central carbon atom connected to four hydrogen atoms in a tetrahedral arrangement, while panel B illustrates the bond angle of 109.5 degrees and bond length of 1.09 angstroms between carbon and hydrogen atoms. — chemical compound Methane, in which four hydrogen atoms are bound to a single carbon atom, is an example of a basic chemical compound. The structures of chemical compounds are influenced by complex factors, such as bond angles and bond length.

The colour of a chemical compound is determined by the specific wavelengths of light it absorbs, which is a direct result of electronic transitions between molecular orbitals (MOs).12

The Mechanism of Colour

When a molecule absorbs light, an electron is promoted from a filled, lower-energy MO to an unoccupied, higher-energy MO. The energy required for this transition ( $Δ E$ ) corresponds to the energy of the absorbed photon, defined by the equation $Δ E = h ν$ .1

Because human perception of colour is the inverse of the absorbed light spectrum, a compound appears to be the complementary colour of the light it absorbs. For example, if a substance absorbs light in the violet-blue region, it will appear yellow to the eye.132

Factors Influencing Absorption

Conjugation: In organic molecules, the presence of conjugated $π$ bonds allows electrons to occupy larger regions of space, lowering the energy gap between MOs. As conjugation increases, the molecule absorbs light at longer wavelengths (lower energy), which often results in intense colouration, such as the orange hue of $β$ -carotene.45
Chromophores: These are specific groups of atoms and electrons within a molecule responsible for light absorption.4
Transition Metals: In coordination compounds, colour often arises from transitions involving the d-level atomic orbitals of the metal ion.2

Compounds that only absorb in the ultraviolet region appear colourless or white because they do not interact with visible light.25