Help with shop items many to many relationship

Hi everybody, I'm working on an e-commerce web where you can buy some items, say shirts... I have three models Item, ItemColor and ItemSize. I have set up a many to many relationship where an item in the shop can have many colors and many sizes available but each of these items will have a stock on it's own, here is where things get complicated for me.

If I create an item, for example a shirt X, I can attach to that shirt different colors and sizes available (many to many), a shirt with black color and size M will have three in stock, but how can I set up so each type of shirt has a stock?? I'm failing to see this in a clear way...

Where should I put stock attribute? in pivot table maybe?

Here are my models and migrations:

//item model
protected $table = 'items';
    
    
    public function colors()
    {
        return $this->belongsToMany('App\Models\ItemColor');
    }

    public function sizes()
    {
        return $this->belongsToMany('App\Models\ItemSize');
    }

//tiemsize model

protected $table = 'item_sizes';
    
    
    public function items()
    {
        return $this->belongsToMany('App\Models\Item');
    }

//item colors

protected $table = 'item_colors';
    
    
    public function items()
    {
        return $this->belongsToMany('App\Models\Item');
    }

And migrations

//pivot table
Schema::create('item_size_color', function (Blueprint $table) {
            $table->increments('id');
            $table->unsignedInteger('item_id')->index();
            $table->foreign('item_id')->references('id')->on('items');
            $table->unsignedInteger('item_size_id')->index();
            $table->foreign('item_size_id')->references('id')->on('item_sizes');
            $table->unsignedInteger('item_color_id')->index();
            $table->foreign('item_color_id')->references('id')->on('item_sizes');
            $table->timestamps();
        });

//items table
Schema::create('items', function (Blueprint $table) {
            $table->increments('id');
            $table->string('title');
            $table->timestamps();
        });

//items color table
Schema::create('item_colors', function (Blueprint $table) {
            $table->increments('id');
            $table->string('title');
            $table->timestamps();
        });

//items size table
Schema::create('item_sizes', function (Blueprint $table) {
            $table->increments('id');
            $table->string('title');
            $table->timestamps();
        });

Mock up controller method, how can I set stock?

public function create(Create $request)
    {
        $item = new Item();
        $item->colors->attach($request->input('availableColors'));
        $item->sizes->attach($request->input('availableSizes'));
        $item->sizes = $request->input('color');
        $title = $request('title');
        if($request->hasFile('image')){ $item->image = $this->uploadFile($request, 'image'); }
        $item->save();

        $item->load('sizes');
        $item->load('colors');

        
        return response()->json([
            'item' => $item,
        ]);
        
    }