onclick

css/style.css

* {
margin: 0;
padding: 0;
}
#h0 {
text-align: center;
}
#d0 {
width: 900px;
margin: 10px auto;
text-align: center;
}
button {
width: 100px;
height: 30px;
border-radius: 3px;
cursor: pointer;
}
#b0 {
border: 1px solid #F00;
}
#b1 {
border: 1px solid #0F0;
}
#b2 {
border: 1px solid #00F;
}

* {
margin: 0;
padding: 0;
}
#h0 {
text-align: center;
}
#d0 {
width: 900px;
margin: 10px auto;
text-align: center;
}
button {
width: 100px;
height: 30px;
border-radius: 3px;
cursor: pointer;
}
#b0 {
border: 1px solid #F00;
}
#b1 {
border: 1px solid #0F0;
}
#b2 {
border: 1px solid #00F;
}

js/script.js

function funRed() {
document.getElementById('h0').style.color = '#F00';
}
function funGreen() {
document.getElementById('h0').style.color = '#0F0';
}
function funBlue() {
document.getElementById('h0').style.color = '#00F';
}
window.onload = function() {
document.getElementById('b0').onclick = funRed;
document.getElementById('b1').onclick = funGreen;
document.getElementById('b2').onclick = funBlue;
}

function funRed() {
document.getElementById('h0').style.color = '#F00';
}
function funGreen() {
document.getElementById('h0').style.color = '#0F0';
}
function funBlue() {
document.getElementById('h0').style.color = '#00F';
}
window.onload = function() {
document.getElementById('b0').onclick = funRed;
document.getElementById('b1').onclick = funGreen;
document.getElementById('b2').onclick = funBlue;
}

index.html

<!DOCTYPE html>
<html>
<head>
<meta charset="utf-8" />
<title>New Page</title>
<link href="css/style.css" rel="stylesheet" />
<script src="js/script.js"></script>
</head>
<body>
<br />
<h1 id="h0">Hello World!</h1>
<br />
<div id="d0">
<button id="b0">Red</button>
  
<button id="b1">Green</button>
  
<button id="b2">Blue</button>
</div>
</body>
</html>

<!DOCTYPE html>
<html>
<head>
<meta charset="utf-8" />
<title>New Page</title>
<link href="css/style.css" rel="stylesheet" />
<script src="js/script.js"></script>
</head>
<body>
<br />
<h1 id="h0">Hello World!</h1>
<br />
<div id="d0">
<button id="b0">Red</button>
  
<button id="b1">Green</button>
  
<button id="b2">Blue</button>
</div>
</body>
</html>