<button id="filter">filter(".custom")</button>

<p>Lorem ipsum dolor.</p>
<p class="custom">Lorem ipsum dolor. (class = "custom")</p>
<p class="custom">Lorem ipsum dolor. (class = "custom")</p>
<p>Lorem ipsum dolor.</p>

<script src="https://ajax.googleapis.com/ajax/libs/jquery/3.4.1/jquery.min.js"></script>

body {
    margin: 20px;
    font-size: 18px;
}

button {
    padding: 10px;
    margin-right: 8px;
    margin-bottom: 18px;
    background-color: tomato;
    color: #fff;
    border: none;
    cursor: pointer;
    font-size: 16px;
}

$(document).ready(function() {

    $("#filter").click(function() {
        $("p").filter(".custom").css("background-color", "lightblue");
    });

});