## Color & Background ##
Magento builds its Model Captcha which extends from `Zend_Captcha_Image` class. So, in your case, we should override `Magento\Captcha\Model\DefaultModel` class.

<?xml version="1.0"?>

<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
        xsi:noNamespaceSchemaLocation="urn:magento:framework:ObjectManager/etc/config.xsd">

    <preference for="Magento\Captcha\Model\DefaultModel"
                type="Vendor\Module\Model\DefaultModel"/>

</config>


We need to override `_generateImage` method.

    protected function _generateImage($id, $word)
    {
          ......
          $w     = $this->getWidth();
          $h     = $this->getHeight();
          $fsize = $this->getFontSize();
          ......
    }

We can remove the generation noise:

[![enter image description here][1]][1]

## Custom Font ##

Add new font to the default config:

*app/code/Company/Module/etc/config.xml*

    <?xml version="1.0"?>
    <config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
            xsi:noNamespaceSchemaLocation="urn:magento:module:Magento_Store:etc/config.xsd">
        <default>
            <captcha translate="label">
                <fonts>
                    <arial>
                        <label>Arial</label>
                        <path>CustomFont/Arial.ttf</path>
                    </arial>
                </fonts>
            </captcha>
        </default>
    </config>

Add our custom font: `lib/internal/CustomFont/Arial.ttf`

[![enter image description here][2]][2]

**Take a look: `vendor/magento/module-captcha/Helper/Data.php::getFonts()` to see more details.**


  [1]: https://i.sstatic.net/uZ0zW.png
  [2]: https://i.sstatic.net/3hjY5.png