I definitely don't have a definitive answer since I don't know the medium, setup, strain, etc. From what I can find, the color of the bacteria is yellow, and as you've pointed out, various organic acids would likely cause the pH to drop as the culture continued. My more interesting idea is that free cupric ions chelated by acetic acid may bind to proteins and turn the culture blue/purple through the biuret reaction. There are also various glutamate assays that produce a blue product, but I couldn't find information on how those work. C. glutamicum is also known to produce carotenoids under stress that may result in a purple color, which may be triggered by the final culture conditions.