Changing the color of the slider on one side of the marker

Hi,

Not extensively tested, but you can start with the following script and adjust.

Qualtrics.SurveyEngine.addOnReady(function () {

    var track = this.getQuestionContainer().querySelector('.track');
    var handle = this.getQuestionContainer().querySelector('.handle');

    if (track && handle) {
        function updateTrackColor() {
            var trackWidth = track.offsetWidth;
            var handleLeft = parseFloat(handle.style.left);
            var percent = Math.ceil((handleLeft / trackWidth) * 100);

            track.style.background =
                'linear-gradient(to right, #ccc 0%, #ccc ' + percent +
                '%, #2196F3 ' + percent + '%, #2196F3 100%)';
        }

        new MutationObserver(updateTrackColor)
            .observe(handle, {
                attributes: true,
                attributeFilter: ['style']
            });

        updateTrackColor();
    }
});