I've been trying to make a grass block slab, so I've looked at the vanilla BlockColors class and put that into my mod. When I run my mod, the sides of the slab are black besides the overlay, and the grass parts are still gray.

 

ColorHandler Class:

public class ColorHandler {
	private final java.util.Map<net.minecraftforge.registries.IRegistryDelegate<Block>, IBlockColor> colors = new java.util.HashMap<>();
	
	public static ColorHandler init() {
		ColorHandler colorhandler = new ColorHandler();
		
		colorhandler.register((state, reader, pos, color) -> {
	        return reader != null && pos != null ? BiomeColors.getGrassColor(reader, pos) : GrassColors.get(0.5D, 1.0D);
	    }, RegistryHandler.GRASS_SLAB.get());
	    return colorhandler;
	}
	
	public void register(IBlockColor blockColor, Block... blocksIn) {
		for(Block block : blocksIn) {
			this.colors.put(block.delegate, blockColor);
	    }
	}
}

 

RegisterColors Class, which has been registered to the mod event bus:

@Mod.EventBusSubscriber(Dist.CLIENT)
public class RegisterColors {
	
	@SubscribeEvent
	public static void registerBlockColors(final ColorHandlerEvent.Block event)
	{	
		ColorHandler.init();
	}
	
}

 

Also the grass_slab.json model:

{   
    "parent": "block/block",
    "textures": {
        "particle": "block/dirt",
        "bottom": "block/dirt",
        "top": "block/grass_block_top",
        "side": "block/grass_block_side",
        "overlay": "block/grass_block_side_overlay"
    },
    "elements": [
        {   "from": [ 0, 0, 0 ],
            "to": [ 16, 8, 16 ],
            "faces": {
                "down":  { "uv": [ 0, 0, 16, 16 ], "texture": "#bottom", "cullface": "down" },
                "up":    { "uv": [ 0, 0, 16, 16 ], "texture": "#top",    "cullface": "up", "tintindex": 0 },
                "north": { "uv": [ 0, 8, 16, 16 ], "texture": "#side",   "cullface": "north" },
                "south": { "uv": [ 0, 8, 16, 16 ], "texture": "#side",   "cullface": "south" },
                "west":  { "uv": [ 0, 8, 16, 16 ], "texture": "#side",   "cullface": "west" },
                "east":  { "uv": [ 0, 8, 16, 16 ], "texture": "#side",   "cullface": "east" }
            }
        },
        {   "from": [ 0, 0, 0 ],
            "to": [ 16, 8, 16 ],
            "faces": {
                "north": { "uv": [ 0, 0, 16, 8 ], "texture": "#overlay", "tintindex": 0, "cullface": "north" },
                "south": { "uv": [ 0, 0, 16, 8 ], "texture": "#overlay", "tintindex": 0, "cullface": "south" },
                "west":  { "uv": [ 0, 0, 16, 8 ], "texture": "#overlay", "tintindex": 0, "cullface": "west" },
                "east":  { "uv": [ 0, 0, 16, 8 ], "texture": "#overlay", "tintindex": 0, "cullface": "east" }
            }
        }
    ]
}

 

Thank you!

Maybe use the ColorHandler provided by the event instead creating your own one?