Source code for https://formidable=masterminds.com/change-input-background-color-based-on-other-field-value/

css_file.css

<style type="text/css">
    .color_red {
        background-color: red !important;
        color: #fff !important;
    }
    
    .color_yellow {
        background-color: yellow !important;
        color: #000 !important;
    }
    
    .color_green {
        background-color: green !important;
        color: #fff !important;
    }
</style>

jquery_file.js

<script>
    jQuery(document).ready(function($){
       "use strict";
        /* replace llopn with the number field's key in the field's Advanced section */
       $("#field_llopn").on("change", function () {
            var num_value = $(this).val();
            /* first remove any color classes that have already been applied
             * change h6ub8 to the other field's key in the field's Advanced section
             */
            $("#field_h6ub8").removeClass("color_red");
            $("#field_h6ub8").removeClass("color_yellow");
            $("#field_h6ub8").removeClass("color_green");
            
            /* set the new color class */
            switch (num_value) {
                case "1" :
                    $("#field_h6ub8").addClass("color_red");
                    break;
                case "2" :
                    $("#field_h6ub8").addClass("color_yellow");
                    break;
                default :
                    $("#field_h6ub8").addClass("color_green");
            }
       });
    });
</script>