Joselin,
<!DOCTYPE html>
<html lang="rus">
<head>
<meta charset="UTF-8">
<title>Title</title>
</head>
<body>
<input type="text" class="form-control" id="NameField" placeholder="Значение для вставки">
<input type="text" class="form-control" id="NameField1" placeholder="Значение для вставки">
<select id="choose">
<option value="10">10</option>
<option value="20">30</option>
<option value="30">60</option>
</select>
<input type="button" id="choo" class="button-onclick" onclick="insertAfterBegin(this)" value="Красный">
<input type="button" id="choo1" class="button-onclick" onclick="insertAfterBegin(this)" value="Синий">
<input type="button" id="choo2" class="button-onclick" onclick="insertAfterBegin(this)" value="Зеленый">
<button onclick="insertAfterBegin()">Вставить</button>
<!-- добавляем атрибут событий onclick -->
<div style="border:1px solid orange">
</div>
<script>
const div = document.querySelector("div"),
inp = document.getElementById("NameField"),
inp1 = document.getElementById("NameField1"),
cho = document.getElementById("choose"),
choo = document.getElementById('choo'),
ch = document.getElementById("choo1"),
c = document.getElementById("choo2");
function insertAfterBegin(el) {
if (inp.value) div.insertAdjacentText("afterbegin", inp.value);
inp.value = "";
let color = "";
if (el == choo) color = "#FF0000";
if (el == ch) color = "#0000FF";
if (el == c) color = "#008000";
if (color) div.style.color = color;
}
cho.onchange = function() {
div.style.fontSize = this.value + "px";
}
</script>
</body>
</html>