Final answer:
Cobalt is the transition metal often associated with a deep blue color, resulting from electron transitions within the d orbitals.
Step-by-step explanation:
The transition metal often used to describe a deep blue color is cobalt. Cobalt compounds, like cobalt (II) nitrate, are known for their deep blue coloration, which arises from the d-d transitions that occur when light photons cause an excitation of an electron from a lower-energy d orbital to a higher-energy d orbital. Transition metals like cobalt can form vibrantly colored complexes such as azurite (blue), malachite (green), and proustite (red) which are indicative of their partial d-orbitals in various oxidation states.