j0hnik,
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<html lang="en">
<head>
<meta charset="UTF-8">
<title>Document</title>
<style type="text/css">
body{
transition: .3s;
}
</style>
</head>
<body>
цвет фона <br>
<input type="range" class="color" min="0" max="255" value="255">R<br>
<input type="range" class="color" min="0" max="255" value="255">G<br>
<input type="range" class="color" min="0" max="255" value="255">B<br>
<script>
var col = [...document.querySelectorAll('.color')];
col.forEach(el => el.oninput =e=> document.body.style.backgroundColor = `rgb(${col.map(e => e.value)})`);
</script>
</body>
</html>