Make sure your TV Out card supports TV=Out in DOS Mode (not all do).
Next of all, it sounds like the Green gun is bad (or the green signal), is not getting to the TV somehow. Since most TVs sync on Green, this may be the problem.
I suggest trying a different S-video cable. Also, try to make sure that the Video is set to the correct input (sometimes TVs have a Composite/S-Video input select). If there is no input select, try high or low numbered channels. I suggest trying channels 0, 1, 2, 90, 91, 92, 99,100,125, and 999. Also, try pressing down on the TV or remote from channel 1 or 2, or try pressing up from channel 99, 125, and 999.
The other thing to try is to load windows, and see if it works there.