I have seborrheic dermatitis, treated with triamcinolone ketaconozole cream, hypopigmentation is visible. How do I regain tone of skin (light brown)?

Strong steroid. Triamcinolone is a strong corticosteroid to be treating anything on the face. I would try decreasing to a less potent steroid and moisturizing the area frequently.
Time. With time, your skin may regain its initial look, although, in some cases this doesn't happen.