One of the long-standing facts about LEDs (Light-Emitting Diodes), was that for any color (wavelength) of light to be emitted, a different semiconductor material was needed, each with a different gap-energy, and therefore also each with a different forward voltage-drop. (:1)
What seems to have happened in more recent times, is that in the cheap, mass-production of LEDs, the Industry is realizing cost-reduction, by always producing the famous Blue LED (InGaN), but putting a different phosphor coating onto it, which absorbs most of the blue light, and which emits a mixture of wavelengths to taste.
But this is also the reason for which, when we buy glow-sticks for camping for example, we may no longer find the traditional, deep red glow-sticks as easily as we could before. What has happened is that here too, a Blue LED is used, but a phosphor put onto it that emits red light. And then, a small amount of the blue light passes through the phosphor without being absorbed, so that a ‘pink’ shade of light emerges.
At least, if the ones I just bought are any indication.
One disadvantage to this is the fact that deep-red light tends to blind human vision the least at night, so that assuming night-vision, there used to be advantages to pure red light for signaling purposes. Because the newer LEDs still contain some light-energy at the (shorter) wavelength that looks blue, those LEDs will also have a stronger, undesirable tendency to ‘dazzle’ a person’s night-vision.
My favorite color of glow-stick for camping is actually the green one.