Change you LS374 to an HC374/HCT374, or better an AC374/ACT374.
At the Emitter output of your NPN transistors, add a 75 ohm series resistor before you drive your output color. Without a load on the video out, the video should be 1.5vp-p. When you connect it to a monitor which has a 75 ohm load, the video level will drop to 0.7vp-p.
At the Base add a 2k pull up to VCC. and a 2k pull-down to GND. (You can adjust these 2 to bias the transistors better into their optimum range and change the video brightness) Don't start the video down at 0v, more like 0.2v, otherwise, then dark colors might smear.
And PLEASE, if you are breadboarding this, or have yet to build a pcb, PLEASE change the LS374 to an HC574, or HCT574, or AC 574, or HCT574. (You will thank me for this one...)