Final answer:
In Premiere Pro, a red bar indicates computationally difficult source files, while a yellow bar indicates effects or unrendered clips.
Step-by-step explanation:
When editing in Premiere Pro, if you see a red bar at the top of your sequence, it means that the source files are computationally difficult and may not play back properly. On the other hand, if the bar is yellow, it indicates that the clips have effects applied to them or they are unrendered clips from Adobe After Effects, and they may not play back in real time.