I'm looking through Qt textbook right now, it seems that - as I've mentioned - it is possible through QSS.
Try this:
@
// use your QTextEdit, of course:
QTextEdit test;
test.setStyleSheet("border: rgb(255, 0, 0); ");
@
You can use common colour names, too. For more info, look into documentation:
"Qt Style Sheet":http://developer.qt.nokia.com/doc/qt-4.7/stylesheet.html#id-7e8519ff-eddd-442c-ad11-e4519ccf273a