There is a world shortage of purple balloons.
There is also a growing shortage of purple crepe paper. This, combined with the shortage of purple balloons, make decoration difficult.
I'm not sure if you've noticed, but it's reaching the crisis level. There simply aren't any purple balloons to be found. I'm sure the ones that are left can only be bought on the black market. Which isn't the best market for purple balloons, because the people in need of purple balloons are the mothers of little two-turning-three-year-old girls whose favorite color is purple (the daughters, not the mom) and in order to have a proper birthday celebration, need purple balloons. And these mothers rarely have any knowledge of black market affairs.
I admit, I am such a person. Last week I went to four different stores on my quest. Two of those stores were Super Wal-Marts in two different states. Having no idea where to find a black market, I had to resort to paying extra for three star-shaped purple Mylar balloons. Purple ribbon was the substitute for purple crepe paper. The decorations were unconventional and nothing like what I pictured,. Thankfully, my two-turning-three year old doesn't seemed to have developed a taste for critiquing party decorations. Or maybe she knew of the purple balloon crisis and was just too polite to mention anything.
In any case, if you happen across purple balloons, you might want to buy them and save them in your food storage. In a few years, those things will be worth their weight in gold.