Depends on how your color is setted up in your DATABASE (or in your case → in the INPUT).

$w.onReady(function () {
const bigBox = $w('#box0');
const firstCircle = $w('#box1');
const secondCircle = $w('#box2');
const thirdCircle = $w('#box3');
const fourthCircle = $w('#box4');
const fifthCircle = $w('#box5');
const changeBoxBtn = $w('#changeBox');
const colorInput = $w('#input2') ;
changeBoxBtn.onClick(event => {
bigBox.style.backgroundColor = colorInput;
bigBox.style.borderColor = colorInput;
});
});