let t = 0;
let b1 = <HTMLElement>document.getElementsByClassName("b1")[0],
    b2 = <HTMLElement>document.getElementsByClassName("b2")[0],
    b3 = <HTMLElement>document.getElementsByClassName("b3")[0],
    timer = <HTMLElement>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 as string;
    }
    else if(t<8){
        b1.style["background-color"] = "unset";
        b2.style["background-color"] = "green";
        timer.innerHTML = 8-t as string;
    }
    else if(t<13){
        b2.style["background-color"] = "unset";
        b3.style["background-color"] = "yellow";
        timer.innerHTML = 13-t as string;
    }
}
setInterval(c_timer, 1000);