This tutorial will help you to avoid the color ball flicker + your color balls won't pass the 4:3 outline into the black bars (as in the following image).
The solution to these problems is adding video track with white solid color which has to be on top of all other tracks.
Set the new track you made to: Multiply (mask)
Go to the cropping settings and crop the white solid to 4:3.
The only step left to do is to remember to crop your color balls in 16:9. The white solid we added will keep them from being visible out of the 4:3 border and will prevent the bad flicker.