What is the ruling of plucking grey hair?
According to madhhab Syafie, it is makruh to pluck grey hair. Rasullullah PBUH said:
لاَ تَنْتِفُوا الشَّيْبَ مَا مِنْ مُسْلِمٍ يَشِيبُ شَيْبَةً فِي الإِسْلاَمِ إِلاَّ كَانَتْ لَهُ نُورًا يَوْمَ الْقِيَامَةِ
“Do not pluck out grey hair. If any believer grows a grey hair in Islam, he will have light on the Day of Resurrection.”
Whereas in another narration, Rasullullah PBUH said:
لاَ تَنْتِفُوا الشَّيْبَ مَا مِنْ مُسْلِمٍ يَشِيبُ شَيْبَةً فِي الإِسْلاَمِ إِلاَّ كَتَبَ اللَّهُ لَهُ بِهَا حَسَنَةً وَحَطَّ عَنْهُ بِهَا خَطِيئَةً
“Do not pluck out grey hair. If any believer grows a grey hair in Islam, Allah will record on his behalf a good deed for it, and will blot out a sin for it.” 
Imam al-Nawawi stated the makruh ruling of plucking grey hair is as agreed by Syafi’iyyah scholars and this is affirmed by al-Ghazali al-Baghawi and others. Regardless of whether the grey hair is in one’s beard or on his head, it is still makruh. 
In a narration from Saidina Anas bin Malik radhiallahu anhu, he said:
يُكْرَهُ أَنْ يَنْتِفَ الرَّجُلُ، الشَّعْرَةَ الْبَيْضَاءَ مِنْ رَأْسِهِ وَلِحْيَتِهِ
“It is makruh if a person should pick out his white hair from his head or beard,” 
Imam al-Nawawi when commenting on this hadith said: “Our colleagues (al-Syafi’iyyah scholars) as well as the ashab Malik” said that it is makruh and not haram (prohibited).” 
If such is the ruling, then is it permissible to colour grey hair? It is permissible according to a narration from Jabir bin Abdillah Al-Ansari radhiyallahu ‘anhuma when Abu Quhafah was brought on the opening of the city of Mecca, his hair was all grey. Then the Prophet PBUH said:
غَيِّرُوا هَذَا بِشَىْءٍ وَاجْتَنِبُوا السَّوَادَ
“Change this with something, but avoid black.” 
This hadith clearly shows the permissibility of changing the colour of grey or white hair with colours other than blacks such as by using henna or other permissible matters. While the above hadith also clearly shows the prohibition of changing the colour of grey hair to black and this is the chosen opinion in our madhhab, as stated by Imam al-Nawawi. Wallahua’lam.