^
We can't colour our hair black because the Prophet (saws) said, "Avoid it" and because of the punishments of it described in the hadeeth.
Jaabir ibn ‘Abdullah reported that Abu Quhaafah was brought on the day of the conquest of Makkah, and his head and beard were white like "thaghaamah" (a plant whose flowers and fruit are white). The Messenger of Allah (Peace & Blessings of Allah be upon Him) said: "Change this with something, but avoid black." (Reported by Muslim, no. 3962).
Ibn ‘Abbas (may Allah be pleased with him) said: "The Messenger of Allah (Peace & Blessings of Allah be upon Him) said: ‘At the end of time there will be people who will dye their hair black like the crops of pigeons. They will never even smell the fragrance of Paradise." (reported by Imam Ahmad, see also Saheeh al-Jaami‘, no. 8153).