【三角形の矢印アイコン】borderを指定

<!DOCTYPE html>
<html lang="ja">
<head>
<meta charset="UTF-8">
<meta name="viewport" content="width=device-width, initial-scale=1">
<style>div + div {
  margin-top: 30px;
}
.arrow-explain1 {
  width: 30px;
  height: 30px;
  border-top: 30px solid #cc0000;
  border-right:  30px solid #cccc00;
  border-bottom:  30px solid #00cc00;
  border-left:  30px solid #0000cc;
}</style>
</head>
<body>
<html lang="en">
<body>
<div class="arrow-explain1">
</div>
<script>
</script>
</body>
</html>

div + div {
  margin-top: 30px;
}
.arrow-explain1 {
  width: 30px;
  height: 30px;
  border-top: 30px solid #cc0000;
  border-right:  30px solid #cccc00;
  border-bottom:  30px solid #00cc00;
  border-left:  30px solid #0000cc;
}