Anything could be true, but I have to tell you that I have glow-in-the-dark plastic rosaries, and they do not meet this requirement. I looked at the rosaries that were put out last year at Old Parish, and there is nothing wrong with them (all black or all white, as part of the month of May). I would also think that people who used them properly would not have their prayers invalidated, since they did not intentionally do anything wrong.
Could it have just been a bad plastic manufacturer, or a bad mold?
I remember some evangelical friends of mine about 20-some-odd years ago, who were all in a panic because some models, drawings and other objects which contained rainbows did not have all 7 colors of the rainbow in them. As God made the rainbow one paritcular way, all other representations of the rainbow were “New Age”. They were throwing away, among other items, a very pretty sun catcher and one of their daughter’s Care Bears.