Даниил

<div id="circle" style="width:100px; height:100px; border-radius:50px; background-color:RGB(255,0,0)"></div>
<script>
(function(){
       var div=document.getElementById("circle");
       var lastR=255;
        var lastG=0;
       var lastB =0;
       setInterval(()=>{
               div.style.backgroundColor="RGB("+lastR+","+lastG+","+lastB+")";
               if(lastG<255)
               	lastG++;
               else
               	lastG=0;
               if(lastG<255){
               	lastB++;
               }
               else 
               	lastB=0; 
           },100);
   })();
</script>