const START_BTN = document.getElementById("start")
const TEXT = document.getElementById("top")
let TIMER = document.getElementById("time")

START_BTN.addEventListener("click", start)
TIMER.innerText = "30"

let TIMER_FUNC = setInterval(countdown, 1000);


function start(){
    START_BTN.style.display = "none"
    TEXT.style.display = "block"

    countdown()
}

function countdown(){
    TIMER.innerText--
}

    <div class="game">
        <button id="start">Start</button>
        <div id="top">
            <div class="color1">RED</div>
            <div class="color2">YELLOW</div>
        </div>
        <div class="bottom">
            <div class="row1">
                <button class="blue"></button>
                <button class="yellow"></button>
            </div>
            <div class="row2">
                <button class="green"></button>
                <button class="red"></button>
            </div>
        </div>
    </div>
    <div class="span">
        <span id="score"></span>
        <span id="time"></span>
    </div>