yeah, you need the experimental branch, as mikae posted.
there is a new spectral upsampling option (advanced tab - spectral upsampling), andrea wrote about it extensively here - spektrafilm tech discussions
it is essentially meant to make the foliage more green and the skin tones more warm (memory colors and what not), could definitely change the tone of the grass in your experiment. also, there are some other new options in this version, i recommend experimenting with the film base parameters
(and you dont have to use the command to get the version if you have a local installation of spektrafilm, you can just do it manually - download the code for the experimental branch - Branches · andreavolpato/spektrafilm · GitHub - go to your local installation, remove the three spektrafilm folders inside the site-packages folder and paste in the three folders from the experimental zip file, they are in the /src/ folder)