@SubscribeEvent
public void onChunkLoad(ChunkEvent.Load event) {
    if (!(event.getLevel() instanceof Level level) || !event.isNewChunk()) return;

    if (level.isClientSide()) {
        ChunkAccess chunk = event.getChunk();
        ChunkPos pos = chunk.getPos();
        Vector2i coordinatesToWorld = new Vector2i(pos.x * 16, pos.z * 16);
        Vector2i chunk_origin = new Vector2i(
                Math.floorMod(coordinatesToWorld.x, 512),
                Math.floorMod(coordinatesToWorld.y, 512)
        );
        Vector2i region_coordinates = new Vector2i(
                pos.getRegionX(),
                pos.getRegionZ()
        );
        int[] colors_data = new int[256];

        for (int z = 0; z < 16; z++) {
            for (int x = 0; x < 16; x++)
            {
                int highestY = chunk.getHeight(
                        Heightmap.Types.MOTION_BLOCKING_NO_LEAVES,
                        x,
                        z
                );

                BlockPos blockPos = pos.getBlockAt(x, highestY-1, z);
                BlockState state = level.getBlockState(blockPos);
                int colorsValue = getColorsValue(state);

                System.out.println("block: " + state.getBlock() + ", height: " + highestY);

                colors_data[z * 16 + x] = colorsValue;
            }
        }

        MinecraftServer server = ServerLifecycleHooks.getCurrentServer();

        if (server != null) {
            ExampleMod.MOD_FOLDER.AttemptToModifyRegionArea(
                    chunk_origin.x,
                    chunk_origin.y,
                    16,
                    16,
                    colors_data,
                    new RegionRecord(
                            level.dimension().location().toString(),
                            server.getWorldData().getLevelName(),
                            region_coordinates
                    )
            );
        }
    }
}

Edited 2 hours ago2 hr by Nofraag

I am not sure but

if (level.isClientSide()) { ... }

here always return false (singleplayer is some kind of local server) - so this is used for client-thread stuff like changing GUIs or working with effects

So this code block is completely ignored - to make sure, test it with LOGGER.info() lines or System.out.println()

So if I am correct, just unwrap it from the if (level.isClientSide()) { ... } check

44 minutes ago, TileEntity said:

I am not sure but

if (level.isClientSide()) { ... }

here always return false (singleplayer is some kind of local server) - so this is used for client-thread stuff like changing GUIs or working with effects

So this code block is completely ignored - to make sure, test it with LOGGER.info() lines or System.out.println()

So if I am correct, just unwrap it from the if (level.isClientSide()) { ... } check

no, no it does get reached the problem it's that chunk.getHeight for some reason returns -64 instead of the actual highest block on that position (the line System.out.println("block: " + state.getBlock() + ", height: " + highestY);) keeps printing block: minecraft:void_air, height: -64

hmm I have no idea

If I had to guess, the client side chunk is not populated at the load time. As the server has to sync the data to the client.

Why not just do all of this server side?