The first way is basic HTML / CSS. You add a class to the description in order to give it different styles. Something like :
.my-description {
color: white;
}
The second way is not totally cross-browser, since not every browser supports pseudo-selectors. So if you want this to work back on something like IE6, you'd better go with the first way.
But if you don't mind about legacy browsers, then you could try this :
.my-article p:first-of-type {
color: white;
}
Basically it tells to the first <p> tag in the element .my-article to be white.
Edit: I just dug a little bit on your code, and you can do it very easily without changing anything.
#archive-page-title p {
color: white;
}
Your #archive-page-title div has only one <p> tag in it: the description. You can style it without a problem.
