The Messenger of Allah ﷺ did not reach the point of having enough grey hair to dye, but Abu Bakr used to dye his hair with henna and katam until his hair turned deep red.
حدثنا هشام بن سعيد وهو أبو أحمد الطالقاني حدثنا محمد بن راشد قال سمعت مكحولا يحدث
لم يبلغ رسول الله ﷺ من الشيب ما يخضب ولكن أبا بكر كان يخضب بالحناء والكتم حتى يقنأ شعره
Its isnad is sahih. Muhammad bin Rashid Khuzai was deemed reliable by scholars, and his hadith is found in the collections of the group. The hadith was mentioned previously at 12572. As for his statement "yathfau shaarahu," it means his hair turns dark, what we would call in our dialect "ghamiq" (deep/dark colored).