Conjugated compounds have a smaller gap between their σ and σ* orbitals which is why these compounds can absorb wavelengths of the UV region and thus are light-colored.

Conjugated compounds have a smaller gap between their σ and σ* orbitals which is why these compounds can absorb wavelengths of the IR region and thus are light-colored.

Conjugated compounds have a smaller gap between their π and π* orbitals which is why these compounds can absorb wavelengths of the visible region and thus are light-colored.

Conjugated compounds have a larger gap between their HOMO and LUMO orbitals which is why these compounds can absorb wavelengths of the visible region and thus are light-colored.