I don't get it. Are you measuring the water filter's ability to fulfil its function based on the liquid changing colour rather than a decrease in impurities?
Clean water is colourless, when it passes through the filter, there is no change. If, however, you measured the levels of impurities of the water and cola, before and after passing them through the Brita, it would maybe show better results. A taste test might've worked too.
I'm not sure if it's applicable to use litmus paper to measure the pH of the cola, though it would be interesting if there was a change in acidity after passing through the filter.