Chroma key
Also known as：green screen · colour keying · chroma keying
Chroma key is the technique of making a region of an image transparent based on its colour. For live subtitles, a translation tool renders captions on a pure green background, and OBS keys the green out so only the text remains over the video.
Why live subtitles rely on it
OBS has no native live captioning. The standard approach treats captions as a video source: any program that can draw text renders a window with a solid background colour, OBS captures that window, and a chroma key filter turns the background transparent.
The value is complete decoupling — OBS does not need to know how captions are produced, and the caption program does not need to know anything about OBS.
Choosing the background colour
#00B140 is the film industry’s green screen standard, but it standardises fabric and paint, so a physical backdrop stays even once lit and shot. A caption window is drawn directly by software and has none of those problems, so the value carries no particular advantage here.
The thing that actually bites: OBS’s Key Color Type preset “Green” is 0x00FF00 in the source code. Set the background to #00B140, leave the preset on Green, and OBS keys a colour you never used — forcing Similarity far higher than it should be, at which point text edges start eroding.
So there are only two rules. Both ends must use the same colour, entered as Custom hex or eyedropped directly. And the background hue must sit as far as possible from the text colour: white text with a black outline over green is safe, and captions containing green should key against magenta or blue instead.
Which filter to use
The OBS filters guide separates the two colour-based filters explicitly: Color Key is for graphics and windows, Chroma Key is for green screens and camera footage. A caption window is the former, so try Color Key first.
The order to tune in
Adjust Similarity alone at first, raising it in small increments until the background disappears completely. If edges still carry the background colour, raise Key Color Spill Reduction — the official reference describes it as specifying how aggressively traces of the key colour are removed, with a default of 100, so higher removes more. Getting that direction backwards is common. If the text itself starts vanishing, Similarity has gone too far.
Do not move three or four sliders at once; you will not be able to tell which one did what.