There are four colour models you can use.
For floating-point images, you can use nodes in Batch or the Modular Keyer to create a 16-bit key by extracting a range of colours. See Keyer-HLS Node, Keyer-RGB Node, Keyer-RGBCMYL Node, and Keyer-YUV Node.
In the HLS menu, you set the softness and tolerance ranges using the hue, luminance, and saturation channels.
In the YUV menu, you set the softness and tolerance ranges using the luma and chroma signals of YUV component video.
In the RGBCMYL menu, you set the softness and tolerance ranges using the red, green, blue, cyan, magenta, yellow, and luminance channels. This colour model provides subpixel resolution.