I need to draw several text line in a QPlainTextEdit, with different color.

I have the following class:
Qt Code: 
  1. #include <QDialog>
  2.  
  3. namespace Ui {
  4.     class ProvaColori;
  5. }
  6.  
  7. class ProvaColori : public QDialog
  8. {
  9.     Q_OBJECT
  10.  
  11. public:
  12.     explicit ProvaColori(QWidget *parent = 0);
  13.     ~ProvaColori();
  14.  
  15. private:
  16.     Ui::ProvaColori *ui;
  17.  
  18. public slots:
  19.     void RossoClicked(void);
  20.     void BluClicked(void);
  21.     void updateText(int, const QString &);
  22.  
  23. };
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Qt Code: 
  1. #include "provacolori.h"
  2. #include "ui_provacolori.h"
  3.  
  4. ProvaColori::ProvaColori(QWidget *parent) :
  5.     QDialog(parent),
  6.     ui(new Ui::ProvaColori)
  7. {
  8.     ui->setupUi(this);
  9.  
  10.     connect(ui->RossoBtn, SIGNAL(clicked()), this, SLOT(RossoClicked()));
  11.     connect(ui->BluBtn, SIGNAL(clicked()), this, SLOT(BluClicked()));
  12. }
  13.  
  14. ProvaColori::~ProvaColori()
  15. {
  16.     delete ui;
  17. }
  18.  
  19. void ProvaColori::RossoClicked(void)
  20. {
  21.     updateText(Qt::red, "Red text");
  22. }
  23.  
  24. void ProvaColori::BluClicked(void)
  25. {
  26.     updateText(Qt::blue, "Blue text");
  27. }
  28.  
  29. void ProvaColori::updateText(int col, const QString &s)
  30. {
  32.     tf = ui->plainTextEdit->currentCharFormat();
  33.     tf.setTextOutline(QPen(col));
  34.     ui->plainTextEdit->setCurrentCharFormat(tf);
  35.     ui->plainTextEdit->appendPlainText(s);
  36. }
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don't mind on why updateText() is a slot.

What happen is that text is drown black, but the font is changed.

Where I am wrong?
How can I change the text color?

thanks