The spectral composition of light is a key factor for understanding its visual and non-visual effects, and for evaluating its ecological impacts. Considerable attention has been paid in the last years to the spectral characteristics of different lighting technologies. The spectrum of the light emitted by a lamp, however, may suffer significant changes along the path from the source to the observer. Some of these changes take place abruptly, for instance in reflections, others in a smoother way, like those progressively accumulated during propagation, and all of them contribute to shaping the actual light spectra at different distances from the source. This note briefly recalls the basic physical processes causing these changes and shows some examples of the expected evolution of several indicators, such as the correlated color temperature (CCT), the U500, the radiance per lumen below 500 nm, the RGB components, and the CIE xy coordinates. Three scenarios are considered: a ground based observer monitoring the zenith sky brightness, an observer measuring the direct radiance of the lamps along an horizontal path, and the nighttime lights measured by a satellite in Earth orbit. Awareness of these changes may contribute to a better assessment of the detrimental effects of artificial light in the nocturnal environment.