I need its code representation, like #FFFFFF.
Answer
As of HTML5, the foreground colors of hyperlinks, among other things, are on track for standardization in the form of guidelines for expected default rendering behavior. In particular, taken from the section Phrasing content, the recommended default colors for unvisited and visited hyperlinks are the following:
:link { color: #0000EE; }
:visited { color: #551A8B; }
Notice that there is no recommended default for active hyperlinks (:link:active, :visited:active), however.
You can use these default colors and reasonably expect them to work. But keep in mind that a browser is free to ignore any or all of these guidelines, as it is never required to follow them. It is, however, recommended for a consistent user experience across browsers (which is how "expected" is defined in this context), so chances are that these colors will correspond to the defaults for most browsers. At worst, they still serve as reasonable approximations of the actual values.
In particular, the default unvisited and visited link colors in the latest versions of Firefox and Chrome are consistent with the above guidelines, but recent versions of IE report different values: unvisited links are rgb(0, 102, 204), or #0066CC, and visited links are rgb(128, 0, 128), or #800080. Older versions of Firefox (and possibly Safari/Chrome) had different defaults as well. Those are older versions, however; the main outlier today that I am aware of is IE. No word yet on whether this will change in Project Spartan — currently it still reflects the same values as the latest version of IE.
If you are looking for a standardized color scheme that is used by all browsers rather than suggested by HTML5, then there isn't one. Neither is there a way to revert to a browser's default value for a particular property on a particular element using pure CSS. You will have to either use the colors suggested by HTML5, or devise your own color scheme and use that instead. Either of these options will take precedence over a browser's defaults, regardless of browser.
If in doubt, you can always use the about:blank technique I described before to sniff out the default colors, as it remains applicable today. You can use this to sniff the active link color in all browsers, for example; in the latest version of Firefox (29 as of this update), it's rgb(238, 0, 0), or #EE0000.