Hi ! Dull face may be due to various reasons , it may be your improper diet (though you are feeling that you are taking proper diet, there may be deficiency of some kind) / stress / lack of sleep / lack of hydration / lack of skin care / hormonal imbalance etc.
Think about all the aspects. Try to find the reason/reasons
Next Steps
Work upon the proper solution, if are able to find the reason of your dull face.
Health Tips
Include fresh fruits and veggies in your diet. Keep yourself hydrated. Consult a good dietitian to get your dietary intakes analysed . Try to meditate and take proper sleep of 7 hours.