var t = 0;
var b1 = document.getElementsByClassName("b1")[0], b2 = document.getElementsByClassName("b2")[0], b3 = document.getElementsByClassName("b3")[0], timer = document.getElementsByClassName("timer")[0];
function c_timer() {
    console.log(t);
    t += 1;
    if (t == 13)
        t = 0;
    if (t < 6) {
        b3.style["background-color"] = "unset";
        b1.style["background-color"] = "red";
        timer.innerHTML = 6 - t;
    }
    else if (t < 8) {
        b1.style["background-color"] = "unset";
        b2.style["background-color"] = "green";
        timer.innerHTML = 8 - t;
    }
    else if (t < 13) {
        b2.style["background-color"] = "unset";
        b3.style["background-color"] = "yellow";
        timer.innerHTML = 13 - t;
    }
}
setInterval(c_timer, 1000);