What causes the pattern you see when you hold a strong magnet near a CRT TV? When you hold a powerful magnet near an old-style CRT TV, you see an interesting pattern, which "shows up the field lines" (see https://www.youtube.com/watch?v=_yEu2R1gYSs&feature=youtu.be&t=2m28s ). 
I understand the basics about the general idea why the image is distorted due to electron deflection, as described in http://www.physlink.com/Education/AskExperts/ae521.cfm, but am curious as to why the specific pattern of alternating light and dark regions is seen. Do the electrons follow certain field lines preferentially, leaving other regions of the phosphor screen dark?
 A: Color CRT monitors uses three electron beams, which passes through a mask before hitting the fluorescent layer which either glows red, green or blue. The mask prevents the electron beams, which scans across the inside surface of the screen, from accidentally activating other sub-pixels. This is also illustrated in this image from Wikipedia:

When you bend these electron beams with an additional magnetic field, their paths through the mask change. This causes the beams to fully or only partially activate a sub-pixel and therefore causing darker and bright bands.
If you would repeat this experiment with a black and white CRT monitor you would not see these bands. In that case you would only see the edge of the screen, which lights up, move.
A: I agree with your own response that "electrons follow certain field lines preferentially, leaving other regions of phosphor screen dark."
Here is a link explaining the processes in detail:
Cathode Ray Tube
And the reason it is alternating dark and light is because of varying linear momenta of the incident electrons. As you bend their path, you are changing their velocities too. Changing velocity means changing energies of incidence. Each color corresponds to a particular energy level of escaped photon.
