Having Arduino read a video game screen I want to connect my Arduino to my tv screen while playing a playstation/XBox game. When a certain screen comes up (like say when you die the same screen pops up) then I want Arduino to catch that and then trigger another device on and off. How might I be able to trigger something by having arduino read the screen then trigger something on and off?
Most Arduino platforms contain a very low power embedded processor.  As such it is very unlikely such a device will decode an NTSC or PAL signal let alone modern digitally compressed video.

Instead consider using a light sensor attached to your monitor.  This is more in line with what, for instance, the Arduino Uno is capable of.  If you can identify a spot on the monitor that will only change to a certain brightness at the conclusion of a game, you could look for that brightness level using a light sensor attached to the monitor and connected to an Arduino.

Here is a tutorial about using a light sensor with an Arduino.  Most of the circuit is for the display of which you need not bother with.