Fields associated with a category group
You can access a category's fields exactly like you
access an entry's fields.

{{ category.myLightswitch }}


So this should do it for your categories loop:

{% for entry in entries %}
    {% for category in entry.myCategoriesField %}

        {{ category }}

        {% if category.myLightswitch %}
            EXTRA
        {% endif %}

    {% endfor %}
{% endfor %}