Discrete transitions and characteristic features
Excited gas can radiate at selected wavelengths. An electron changing from a higher to a lower bound state releases a photon associated with the energy difference rather than an equal increase at every wavelength.
Elements and ionization states have different line patterns. Laboratory comparisons identify features, commonly using several mutually consistent lines rather than an image colour alone.
Lines can coexist with a continuum
Real spectra can combine a continuum and lines. NASA illustrates this with the X-ray spectrum of the Cassiopeia A remnant, where elemental features rise above a continuum produced by different processes.
An emission spectrum therefore need not consist only of isolated thin lines. Line diagnostics extend beyond visible light to other bands, depending on the relevant states and transitions.
References: [1]
Position, width and strength
Line strengths, widths and positions respond to conditions and motion. Weakness does not uniquely indicate low abundance: excitation and other physical conditions affect production and visibility.
A feature’s displacement relative to its reference also informs motion or redshift. Analysis separates central wavelength, width and strength while accounting for instrumental resolution.