minecraft-assets/assets/minecraft/shaders/post/desaturate.json

{
    "targets": [
        "swap"
    ],
    "passes": [
        {
            "name": "color_convolve",
            "intarget": "minecraft:main",
            "outtarget": "swap",
            "uniforms": [
                {
                    "name": "Saturation",
                    "values": [ 0.2 ]
                }
            ]
        },
        {
            "name": "blit",
            "intarget": "swap",
            "outtarget": "minecraft:main"
        }
    ]
}