It is not possible to set color of visited hyperlinks in a QTextBrowser, despite the documentation indicating otherwise. QPalette has the Link and LinkVisited color roles, but the documentation for these say:
"Note that we do not use the Link and LinkVisited roles when rendering rich text in Qt, and that we recommend that you use CSS and the QTextDocument::setDefaultStyleSheet() function to alter the appearance of links."
However, the CSS support in Qt does not support the :visited pseudo-class, so it is still not possible to set the color of visited hyperlinks.
Note: This requires also new API for querying of the status of a link (visited or not). Also note that QtWebKit in Qt 4.4 provides API for this.