If the bruise comes and goes, TURN YOUR CONTRAST SETTING DOWN.
What's happening is that full white images draw maximum current and it heats up the internal
component of the picture tube known as a shadow mask, or an aperture grille if it's a Trinitron tube.
This heating causes it to expand slightly, and that causes the electron beam that passes through its millions of tiny holes (or slots) to land partly on the wrong color phosphor.
Your brightness and contrast settings should really never be much above half way. Damage to the picture tube can result.
The shadow mask or aperture grille could be physically misaligned, too. No cure for that but a new tube.
Magnetic distortion is a possibility, too, but trying to manually iron out the bruise with a magnet in your hand is not something you should try. I can do it, but I'm experienced. First, things first...move speakers away from the TV set (and anything else that might be magnetic) and see what happens.
CJ