Effect picture
<!DOCTYPE html>
<html>
<head>
<title></title>
<style>
div {
position: absolute;
left: 35%;
top: 40%;
}
input {
width: 400px;
height: 50px;
}
</style>
</head>
<body>
<div>
<input id="range1" type="range" min="0" max="255" oninput="changeColor();"></input></br>
<input id="range2" type="range" min="0" max="255" oninput="changeColor()"></input></br>
<input id="range3" type="range" min="0" max="255" oninput="changeColor()"></input></br>
</div>
</body>
<script>
function changeColor(){
var value1 = document.getElementById("range1").value;
var value2 = document.getElementById("range2").value;
var value3 = document.getElementById("range3").value;
var color = "rgb(" + value1 + "," + value2 + "," + value3 + ")";
document.body.style.backgroundColor = color;
}
window.onload = changeColor;
</script>
</html>