Show entries by a certain category AND certain lightswitch
You can add the lightswitch filter to the entries query like so:

{% set category = craft.categories.slug('copier') %}
{% set entries = craft.entries()
                      .section('products')
                      .relatedTo(category)
                      .color(true)
                      .all() %}

{# If there are any entries, loop through them #}
{% if entries | length %}

    {# For each entry we find, do this #}
    {% for entry in entries %}
        &lt;p&gt;{{ entry.title }}&lt;/p&gt;
    {% endfor %}

{% endif %}