How can I set the color for the progress element?

You can style the color of the bar in the <progress> element by changing the background of a few browser-proprietary selectors.

In Firefox, you can use the following:

progress::-moz-progress-bar { background: blue; }

In Chrome or Safari, you can use:

progress::-webkit-progress-value { background: blue; }

In IE10, you just need to use the color attribute:

progress { color: blue; }

Source: http://html5doctor.com/the-progress-element/

Altogether, that makes:

progress::-moz-progress-bar { background: blue; }
progress::-webkit-progress-value { background: blue; }
progress { color: blue; }

<progress value="0" max="100"></progress><br>
<progress value="25" max="100"></progress><br>
<progress value="50" max="100"></progress><br>
<progress value="75" max="100"></progress><br>
<progress value="100" max="100"></progress><br>