My son is 4, he had a cavity they had to put a cap on

before they started I asked if it was going to be a white cap to match his other teeth and they said yes, after we got home i noticed it was a grey cap. He goes back to dentist today I was going to ask if they could replace the grey and do white but before I do I was wanting to know if it’d really even be worth it? Would it hurt him worse? He sat through the work the whole time very good. It is a back tooth he will keep until hes 9 or 10 and it is noticeable when he smiles big and laughs..