Most newer CRTs degauss when you turn them on (that's the wierd *boing!* sound you hear before the screen lights up).

There are ways to DYI degauss using a solder gun. Do an internet search.

I know this because my center channel speaker came in unshielded by mistake. When I turned on my TV the lower 1/3 turned green & purple.

Instead of sending the speaker back (shielding drivers negatively alters sound quality according the speaker mfgr), I just placed 2 or 3 layers of sheet metal between the speaker and my TV. I used scrap HVAC duct metal wrapped in a towel and placed on top of my speaker. In my case, it's hidden by my rack. I could have also slid it under the TV (minus the towel). My TV degaussed after cycling the power and everything has been cool since. Gotta love the internet...