<p class="choice">
        <label>
            <input type="radio" class="a" name="1"><span class="aFont">aaaaa</span>
        </label>
        <label>
            <input type="radio" class="b" name="1"><span class="bFont">bbbbb</span>
        </label>
        <label>
            <input type="radio" class="c" name="1"><span class="cFont">ccccc</span>
        </label>
        <label>
            <input type="radio" class="d" name="1"><span class="dFont">ddddd</span>
        </label>
    </p>
<script>
        $(document).ready(function(){
            $(".choice input[type = 'radio']").on("click",function(){
                if ($(".choice input:checked")){
                    $(this).parent().css("background" , "green").siblings().css("background","red");
                }
            });
        });
</script>

The one you selected is input, not label. Note $(this).parent().css("background","green").siblings()

The code has not been tested. The idea is to remove all background colors first, and then add a background color to the clicked one. Your if judgment is not needed

<script>
        $(document).ready(function(){
            $(".choice input[type = 'radio']").on("click",function(){
                   $(".choice input[type = 'radio']").css("background","");
                   $(this).css("background","red");
            });
        });
</script>