My best wrong answer is that there were different colored pixels for different front-end versions, and the app had some radically different responses depending on the version. Maybe MENA would return white, SE Asia green, people who signed up during a sale would return blue, whatever. After a while, the other pixels were removed and only one shade of purple were used for everyone, but the code for processing them was not removed. So now, if the tracking pixel is not a precise shade of purple, some unexpected shenanigans ensue.
I worked on an app once that used two different, equally ancient libraries to a) generate thumbnails and b) create a png from a pdf. While modifying part of this process I started realizing that there were conditions where you'd get a PDF thumbnail at the end, but its output had a red tint to it.
Input looked fine and invoking each step manually worked fine as well.
Come to find out that certain PDFs contained color calibration information that, combined with how we were calling it, would treat ARGB as RGB. The input would have transparency info defined and the thumbnail generator would happily repurpose the alpha channel as the red channel instead.
Now I'm really curious, is there some exciting non-obvious reason for a tracking pixel to be purple? Was it #FF00FF or more like #6600DD?