change colour onclick code example

Example 1: css onclick change color

<style>
.dabutton:focus {
    background-color:yellow;
}
</style>
<button class="dabutton"></button> // <-- usage

Example 2: change color by click event javascript

var button = document.querySelector("button");

button.addEventListener("click", function() {
    const curColour = document.body.style.backgroundColor;

    document.body.style.backgroundColor = curColour === 'red' ? 'blue' : 'red';
});