Yellow into Red

That line is a way of Premiere saying what kind of playback might be likely. Green means it expects no problems. Yellow means it might have problems. Red means it probably will have problems in playing back that section without dropped frames or stuttering.

So ... typically hitting the "enter" key will cause Premiere to make quick previews of any red sections of the timeline.

Or you might choose to make proxies, do a render & replace to bake the effects into a new clip, several other things.

Neil