The progress bars have three colors:

1) Yellowgreen (Learning stage) : Before you reach 100% learned in the content. 

2) Green (Completed stage) : When you reach 100% learned. 

3) Yellow (Review stage) : After you've reached 100% learned content, the progress goes down according to the SRS algorithm. At that time we show the yellow color so that you make it green again by reviewing it and reaching 100% again. You can do this by visiting the Ready for Review section regularly.

When you reach 100% again, the video or audio will be "mastered".