QString style_0_60      = "QProgressBar::chunk{ background-color: green; }";QString style_60_80     = "QProgressBar::chunk{ background-color: orange; }";QString style_80_100    = "QProgressBar::chunk{ background-color: red; }";

#include <QProgressBar>

class MyProgressBar : public QProgressBar{public:
    MyProgressBar();
    void    setValue(int value);};

void MyProgressBar::setValue(int value){
    if(value >= 0 && value < 60)
        this->setStyleSheet(style_0_60);
    else if (value >= 60 && value < 80)
        this->setStyleSheet(style_60_80);
    else
        this->setStyleSheet(style_80_100);

    QProgressBar::setValue(value);}