How to mask a mp4 video with another alpha mp4 video A noob question. I have two mp4 video files. The first one is ordinary color video (right side). 

The second one is transparency mask video (alpha), it's a mp4 video not an image (left side). 
This second video has gray-scale colors, and it changes frame by frame. This second video has no alpha channel. 



What video editor is able to mask the first color video with the second alpha video, so that the first video is rendered with its transparency changing frame by frame according to the second alpha video? And please a short explanation on how to do it? 

I prefer a free editor, but if there isn't one, a proprietary editor doesn't matter. 
Thank you.
This would be easy to do with After-Effects. Do the following:

Create a new composition with the settings of the mp4. You can drag it on the composition symbol to automatically create it.

Then, drag your alpha-mp4 above the color-mp4. Now, set the "track matte" to "Luma-Mask" from the alpha-mp4. This should be all.