The grey ring indicates that the bulb is nearing the end of it's life - all chrismas light bulbs slowly get that while in use. I have noticed wide variation in just how dark the grey ring is at the time of bulb failure, but no apparent pattern, other than that the longer the bulb has run, the darker the ring.
If they're starting out with no ring, but getting one within timescale of hours, you are driving them too hard, apply a lower voltage, or put more in the string.
In the US (120V mains), christmas light strings have one or more series strings of lights. The strings are generally either 35 or 50 lights long (3.5v or 2.5v bulbs).
Once christmas lights get to a certain age, they become more fragile, and will fail upon handling (these will typically have the grey ring at least faintly visible in most or all of the bulbs). Often these will fail without the shunt working, making the entire string go dark.
Always replace bulbs immediately when they fail. Spare bulbs are not readily available at sane prices; I've resorted to buying extra strings for bulbs. When the bulb fails, the shunt makes contact to bypass the failed filiment so the rest of the string stays lit - but this results in a higher voltage across the other bulbs, and the influence of applied voltage on bulb lifetime is huge.
(I use christmas lights for general illumination, and have since I got my own apartment after graduation nearly a decade ago. I replace the bulbs (have gotten really good at swapping the plastic bases), and would save the dead ones (sans plastic base) - when I threw it out had a 2.5" x 2.5" x 10" drawer full of burned out bulbs. I still have christmas lights. I no longer keep the burned out bulbs).