[рони]

ren2222,
решение: запоминать активный элемент.

let model = document.querySelector('h2').innerHTML;

        let aly = document.getElementById('aly');
        let sand = document.getElementById('sand');
        let man = document.getElementById('man');
        let mag = document.getElementById('mag');
        let dark = document.getElementById('dark');

        const colors = [aly, sand, man, mag, dark];

        const bigPic = document.getElementById("big");
        bigPic.style.backgroundImage = 'url(images/' + model + colors[0].id + '.jpg)';
        let temp = colors[0];
        for (let i = 0; i < colors.length; i++) {
            colors[i].onclick = function () {
                bigPic.style.backgroundImage = 'url(images/' + model + colors[i].id + '.jpg)';
                temp.classList.remove('active');
                temp = colors[i];
                temp.classList.add('active')
            };
        }