Atomic line spectra are a series of colored lines that are revealed when the atom is brought to a relaxed state from an excited state.
The colored lines occur due to the fact that electrons give off energy in the form of photons when moving from higher to lower energy level. These photons are in the visible light range and are seen as colors. The line spectra are unique to the element because each element has its own electronic configuration, since it has its own number of electrons.