I learnt that when you want to stitch photos together, these are the things you must follow:
a. Shoot in manual, set the aperture and shutter speed constant throughout your shots.
b. Use manual focus only.
c. Overlap at least 25% with next frame.
d. Use tripod.
e. Must shoot all of them within the short period of time.
I think your photos are shot with different exposures which explain the difference in tone. Are you sure the settings of aperture and shutter speed remain the same throughout the shot?