Oh, I just realised the problem, your model files are missing important things.
"faces": {
"down": { "cullface": "down" },
"up": { "cullface": "up" },
"north": { "cullface": "north" },
"south": { "cullface": "south" },
"west": { "cullface": "west" },
"east": { "cullface": "east" }
}
Each of the faces ("down" etc) needs to contain a "texture" element - a string referring to the texture label (as defined in the "textures" section at the start of your file) and starting with #. You likely also want a "uv" element - an array of four numbers - to define which area of the texture to apply to the face. Here is a random example from a model in my mod:
"faces": {
"down": { "uv": [ 15, 14, 0, 2 ], "texture": "#cotton1" },
"up": { "uv": [ 1, 4, 16, 16 ], "texture": "#cotton1" },
"north": { "uv": [ 0, 15, 15, 16 ], "texture": "#cotton1" },
"south": { "uv": [ 1, 15, 16, 16 ], "texture": "#cotton1" },
"west": { "uv": [ 2, 15, 14, 16 ], "texture": "#cotton1" },
"east": { "uv": [ 2, 15, 14, 16 ], "texture": "#cotton1" }
}