Change the color of radio button (before selection) but only for the last response.

This is a little more tricky than the matrix one, as there Qualtrics would assign the class last to the last statement. But the following code should work. It should work for all combinations of single/multiple choice with horizontal/vertical display.
Qualtrics.SurveyEngine.addOnReady(function () {
    ch = this.getChoiceContainer();
    all_buttons = ch.querySelectorAll("[class^=q-]");
    all_buttons[all_buttons.length - 1].id = "last_button";
    last_button = ch.getElementsBySelector("#last_button")[0];

    // Set border to red
    last_button.style.borderColor = "red";

    // Check for a click in the question
    this.questionclick = function () {
        // If selected, set backgroundColor to red, else clear
        if (last_button.className.includes("checked")) {
            last_button.style.backgroundColor = "red";
        } else {
            last_button.style.backgroundColor = "";
        }
    };
});
Standard caveat: Performance will depend on the theme used. And if you use it multiple times on a page, you'll need to change the variable names and ids.