Something like this:
var format:TextFormat = new TextFormat();
format.color = 0x555555;
format.size = 18;
format.align = TextFormatAlign.CENTER;
format.bold = false;
tf.setTextFormat(format);
Check class reference for TextFormat: http://help.adobe.com/en_GB/FlashPlatform/reference/actionscript/3/flash/text/TextFormat.html