This article is from the Apple II Csa2 FAQ, by Jeff Hurlburt with numerous contributions by others.
It looks like you should be able to do a full-screen color-killer on a IIe by using a general purpose NPN transistor (2N3904, 2N2222, etc.) to shunt the Color Burst signal. The transistor's collector would connect to the junction of R15 and R13; the emitter lead would go to Ground. The transistor should be connected directly to the R15-R13 junction and ground. The base lead would go through a 2k-3k Ohm resistor to an annunciator output, such as An-3 at pin 12 on the 16-pin J-15 Game connector. PEEKing the appropriate addresses should flip Color OFF and ON. The reason for qualifiers such as "looks like", etc. is that I have not actually tried a mod like this on a IIe. Monitors are remarkably sensitive to Color Burst. If the transistor does not do a good job of shunting the signal, enough may get through to trigger Color-ON. A more positve (but less easy) technique is to use a 74LS32 OR gate to control flow of the 3.58MHz signal. The IC could be tack-soldered onto U88 via its +5 and GND pins. U88 is the 74S02 which has the gate which produces Color Burst. Pin 12 of the 'S02 would be bent up and connected to the output of an 'LS32 OR gate (e.g. pin 3). One of the OR gate's inputs (pin 2) would go to Pin 12 of the 'S02 socket. The other (pin 1) would go to an annunciator output. Turning ON the annunciator forces the OR gate output to "1" and eliminates Color Burst. Turning OFF the annunciator lets the OR gate output follow the 3.58MHz signal and enables Color Burst. By: Rubywand ----------------------------