InstallData.php

<?php
/**
 * Copyright © Magento, Inc. All rights reserved.
 * See COPYING.txt for license details.
 */
namespace Magento\Swatches\Setup;

use Magento\Framework\Setup\InstallDataInterface;
use Magento\Framework\Setup\ModuleContextInterface;
use Magento\Framework\Setup\ModuleDataSetupInterface;
use Magento\Eav\Setup\EavSetupFactory;
use Magento\Eav\Setup\EavSetup;

/**
 * Class InstallData
 */
class InstallData implements InstallDataInterface
{
    /**
     * EAV setup factory
     *
     * @var EavSetupFactory
     */
    private $eavSetupFactory;

    /**
     * Init
     *
     * @param EavSetupFactory $eavSetupFactory
     */
    public function __construct(EavSetupFactory $eavSetupFactory)
    {
        $this->eavSetupFactory = $eavSetupFactory;
    }

    /**
     * Install new Swatch entity
     *
     * @SuppressWarnings(PHPMD.UnusedFormalParameter)
     * @param ModuleDataSetupInterface $setup
     * @param ModuleContextInterface $context
     * @return void
     */
    public function install(ModuleDataSetupInterface $setup, ModuleContextInterface $context)
    {
        /** @var EavSetup $eavSetup */
        $eavSetup = $this->eavSetupFactory->create(['setup' => $setup]);

        /**
         * Install eav entity types to the eav/entity_type table
         */
        $eavSetup->addAttribute(
            'catalog_product',
            'swatch_image',
            [
                'type' => 'varchar',
                'label' => 'Swatch',
                'input' => 'media_image',
                'frontend' => 'Magento\Catalog\Model\Product\Attribute\Frontend\Image',
                'required' => false,
                'sort_order' => 3,
                'global' => \Magento\Eav\Model\Entity\Attribute\ScopedAttributeInterface::SCOPE_STORE,
                'used_in_product_listing' => true
            ]
        );
    }
}