Create a rule for h3 tags and inside the braces set the color: green;.

<html>
<head>
<style>
h3 { color: green; }
</style>
</head>
<body>
<h1>Contents</h1>
<h3>The etymology of the word green</h3>
<h3>Where do green pigments come from</h3>
<h3>The wavelength of the color green</h3>
</body>
</html>]]>

Make sure to pick h3 followed by { and then color: green;, }.