Polymer optical fibers doped with organic dyes can be used as efficient optical amplifiers and lasers in the visible region. We computationally analyze the spectral features of both their fluorescence and their amplified emission for different amounts of overlap between the emission and absorption spectra of the dyes employed. Representative cases are compared by calculating the respective evolutions along the doped fiber of the average wavelength, peak wavelength, and full-width at half-maximum of the output power obtained.