@ajbmorais I think the best option is to use shaders, there are several that come in Recalbox and you can test it, just go to the main menu, game options, shaders.
If you want, you can download new Shaders from the internet, I believe the "grayAverage" shader is what you are looking for:
https://github.com/aybe/RetroArch-shaders
See if this helps:
https://youtu.be/_R0yvMB5zIk?t=277 https://recalbox.gitbook.io/documentation/v/francais/tutoriels/jeux/generalite/configuration-des-shaders
