minecraft-assets/assets/minecraft/blockstates/mycelium.json

{
    "variants": {
        "snowy=false": [
            { "model": "block/mycelium" },
            { "model": "block/mycelium", "y": 90 },
            { "model": "block/mycelium", "y": 180 },
            { "model": "block/mycelium", "y": 270 }
        ],
        "snowy=true": { "model": "block/grass_block_snow" }
    }
}