Answer:
Because their unfilled d-orbital becomes excited
Step-by-step explanation:
Transition metals are a group of elements that has an incompletely filled d shell e.g chromium, nickel, copper, iron etc. This characteristics of transition elements confers upon them the ability to form colored ions or compounds.
Transition elements/metals form colored compounds when the partially complete d-orbital becomes excited as a result of absorption of visible light. d-orbital at its lower energy becomes excited to d-orbital at a higher energy.