Understanding colors in WebPageTest waterfall view

Question

I'm using webpagetest.org and am struggling understanding the meaning of the different colors in WebPageTest waterfall view. As you can see in the image below, each type (HTML, JS, CSS, etc.) has two colors – the former being paler than the latter. Anyone knows what is the meaning of these two colors?

Answer 1

According to this post from one of the authors:

The lighter time is the time from when the browser sends the request out on the wire until the first byte comes back. The darker color is the period where the resource is actually downloading.