Some people are attached to the color green. They always wear green hats, attributing their choice to the color of the dome of the Prophet's mosque. Is there any significance to the green color?
The color green in the Qur`an and Sunnah
The color green is a symbol of prosperity and good life. It is mentioned in the noble Qur`an as the color of the garments and cushions of the people of paradise. God Almighty says,
“Upon them will be green garments of fine silk and heavy brocade” [Al-Insan: 21], “And they will wear green garments of fine silk and heavy brocade” [Al-Kahf: 31], and “Reclining on green cushions and beautiful fine carpets” [Al-Rahman: 76].
The Prophet (peace and blessings be upon him and his family) said that God will clothe him in green on the Day of Judgment as per his words, "The people will be resurrected on the Day of Judgment, and my community and I will be on a hill. God Most Exalted will then clothe me in a green garment and permit me to supplicate Him and I will say whatever He wills I should say; this is the station of praise and honor (al-Maqam al-Mahmud)" [recorded by Imam Ahmed].
The Prophet (peace and blessings be upon him and his family) also said that the souls of martyrs are kept in the crops of green birds.
The dome of the Prophet's mosque was painted green in 1253 A.H.
The green flag
Al-Khara`iti reported from the hadith of Asma` bint Abu Bakr (may God be pleased with her) who narrated that Zayd ibn Amr ibn Nufayl went out the night of the Prophet’s birth. When he reached Mount Abu Qubays, he saw a man with two green wings descend on the mount. The man looked down on the city of Mecca and said, "Satan has been humiliated, the idols abolished and the Ameen [trustworthy] born."
Filled with alarm, he saw the man spread a piece of cloth from the east to the west that covered everything underneath the heavens and emanated a light so splendid that it almost blinded him. The man fluttered to the Ka'ba and a brilliant light shone [the city] of Tihama. He then said, "The earth has been blessed and flourished." He pointed to the idols on the Ka'ba and they all fell from their places. This report was mentioned by al-Salihi in Subul Al-Huda Wal Rashad.
Other signs were seen on the night of the Prophet's birth such as the glad tidings brought by the jinn and the shining of light.
The color green in modern studies
Modern scientific experiments prove that green has a positive effect on an individual’s psychological state. Green is a pleasing color is and it is commendable to be attached to it for the above reasons.
The right thing to do is to let people behave according to their natural disposition with regard to their expression of love for the Messenger (peace and blessings be upon him and his family) and by way of attaching themselves to the color of the garments of the inhabitants of paradise and believing in the unseen.
To consider this attachment a bid'a [innovation in religion] is a bid'a in itself because it entails restricting what Islamic law has left unrestricted on the one hand and preventing people from expressing their love on the occasion of the Prophet's birth (peace and blessings be upon him and his family) on the other.
God Almighty knows best.