In Ibn Hajar Haytami's list of "enormities", item 111 is:
Dying the beard or hair black for other than jihad or the like.
(Also mentioned at Islam Q&A.)
Question: Does dying one's beard or hair black play a role in jihad?
I'm not sure how reliable this enormity is. The answers to Can I dye my beard or hair black? don't list jihad as an exception to the ruling against dying one's hair and/or beard black.