Hi,
So you'll end up with a black and white FBO with your trails, if you draw that on top of your video with additive blending the black will become transparent like so:
(Just nicked this from the Internets, it shows individual particles being drawn so it's not the same, but you can see the additive blending in effect )
If you have a color other than white set (via ofSetColor) when your draw your trail FBO on top of the video, the trail FBO will use that color instead of white.