Answered by Shaykh Faraz Rabbani
From my understaning their is a hadith that the beared must not be coloured back to its original colour, i.e. if one has balck hair and due to age they are now grey then they must not be dyed to black, if an Imam does this would it still be permissible to read salah behind him?

There is difference of opinion regarding this matter. Therefore, one would not condemn someone who colors their beard black–though, if appropriate, one may advise them–nor avoid praying behind them.
Fiqh of Hair Dyes
Faraz Rabbani