thank you for your query,
side effects of TB drugs include skin "yellowing".if that is the case you need to contact your doctor.
otherwise skin darkening can be due to stress
and irregular sleep cycle
associated with the long drug regimeN
In the meantime you can follow these tips
1.regularly clean,tone and moisturize your skin
2.drink plenty of water.
3. follow proper sleep and food timings
wishing you good health