Here’s another interesting callout sample where each callout is denoted by a different color. Frankly, the implementation is not too great — I don’t like the choice of some text and callout colors which are too light — notice the pink right at the bottom — we had to Photoshop it a bit to make it visible — even the results don’t look too great with this color. However, the general idea works well — and this is certainly an interesting way of using callouts.
This example is from Hello 6E, the inflight shopping magazine on IndiGo, an Indian airline.
What could make this implementation better is if the colors used for the callouts had a similar color intensity. If you could do so within your colored callouts, then this is one of those treatments that would work well within a PowerPoint slide — do make sure though that the text size you use is big enough so that everyone within your audience can read it well.