The shade can definitly be modified by a competent technician. however, addressing your concern to your dentist is the best way to achieve optimal outcome.
I am having a front crown and a lower bridge re-manufactured for the third time. Each previous attempt the result has been too creamy. The lab seem incapable of resolving this despite a personal visit. I am visiting the lab again tomorrow to give this one last try. I regret removing the original as it's colour was closer than anything the lab has achieved. I work with engineering suppliers for a living and would have terminated this co. by now.
You would have to have new crowns matching the color of your surrounding teeth. This should have been taken into consideration by your dentist when choosing the color of your crowns.