A light bulb includes a base for mechanically mounting the light bulb and for receiving electrical power, a light guide, and a light source that directs output light into the light guide. The light source includes a light emitting device that emits light, and a variable spectrum adjuster that is variably positionable relative the light path of light emitted by the light emitting device. The spectrum adjuster includes a region of continuously-variable spectrum-adjusting material, usable for adjusting the spectrum of light passing through the spectrum adjuster. In some embodiments, the spectrum adjusting material is a color-attenuating material, such as a filtering material. In other embodiments, the spectrum-adjusting material is a wavelength-shifting material, such as a phosphor, or another suitable type of material that shifts the wavelength of light incident.