How to mask a mp4 video with another alpha mp4 video
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.