TLDR: DO YOU WANT RGB?Just add this to the HTML of your site.
<script>
var dg = 0
const speed = 10
setInterval(() => {
// Hey you can modify the querySelector
document.querySelector(':root')
.style.setProperty('filter', `hue-rotate(${dg}deg)`);
dg += speed
}, 100);
</script>
This will not affect your background image, and that's too bad.