There are many reasons you could have lost your color. If you were using the S4
PAR EA fixtures, and they were being used as up-light, all the heat from the lamp and the IR and UV radiation were passing through the
gel. You would have nearly 100% of the gel damaging heat and radiation hitting the gel. The S4 PAR
MCM would have removed some of the radiation from the light and may have saved you some gel, but since heat rises, you would still be directing the rest through the gel. Also, if you picked a "nice" purple, it was probably very saturate, with a low transmission rate, so the chances of it burning through are greatly increased, especially because you probably ran the lights at very near to full intensity.
I doubt it was a case of old gel. If your local supplier really keeps 20+ sheets of each color in stock all the time, there is probably a high demand for color, and they probably sell it. Also, I have a huge color inventory at my
theatre, much of it has been sitting around for years and years, and it doesn't seem to matter if color is new or old, if the color is going to burn through, it happens to the new and the old alike. In my experience anyway.