An emitter spews out spheres that start out dark grey and when passing boundaries at the left and right, change colour as they cross the boundaries.
The job is fairly straight forward because Blender's particle system reproduces the same particle pattern and animation each time. It means you can render out three separate videos and the particles in all three will move identically.
Use a mask in front of the camera (or do it with masking nodes) that's divided vertically into 3 segments. One each for left & right segments and one for the center.
Remove one face at a time and set the the emitter object's colour, (sphere) to whatever you like.
Render that with a transparent background. (alplha layered)
Replace the first face, remove the next and change the sphere color. Render that out with a transparent background also. (png with RGBA active?)
Do the same a third time with the last face removed.
Mix all three in the VSE using "Alpha Over" in each strip's properties, and add a suitable background on the very bottom strip.