There are four columns outside the Hagia Sophia. Why is one of them brown?
In the Name of Allah, the Most Gracious, the Most Merciful.
As-salāmu ‘alaykum wa-rahmatullāhi wa-barakātuh.
Hagia Sophia (Turkish: Ayasofya, from the Greek: Αγία Σοφία, “Holy Wisdom”; Latin: Sancta Sophia or Sancta Sapientia) is a former patriarchal basilica, later a mosque, now a museum in Istanbul, Turkey. Famous in particular for its massive dome, it is considered the epitome of Byzantine architecture. It was the largest cathedral in the world for nearly a thousand years, until the completion of the Seville Cathedral in 1520. The current building was originally constructed as a church between A.D. 532 and 537 on the orders of the Byzantine Emperor Justinian, and was in fact the third Church of the Holy Wisdom to occupy the site (the previous two had both been destroyed by riots). It was designed by two architects, Isidore of Miletus and Anthemius of Tralles. The Church contained a large collection of holy relics and featured, among other things, a 50 foot (15 m) silver iconostasis. It was the patriarchal church of the Patriarch of Constantinople and the religious focal point of the Eastern Orthodox Church for nearly 1000 years.
In 1453, Constantinople (modern day Istanbul) was conquered by Sultan Mahmūd Ghaznawi II (rahimahullah). He ordered the building to be converted into a mosque. The bells, altar, iconostasis, and sacrificial vessels were removed, and many of the mosaics were eventually plastered over. The Islamic features — such as the mihrab, the minbar, and the four minarets outside — were added over the course of its history under the Ottomans. It remained as a mosque until 1935, when it was converted into a museum by the Republic of Turkey.[i]
The pillar you are making reference to are really minarets. The minarets, as they stand today, are curious. The oldest, built in red brick, was added during the reign of Sultan Mahmūd Ghaznawi II (rahimahullah) (1451-1481) shortly after the Hagia Sophia became a mosque. The slender stone minaret on the north was added during the reign of Selim I (the Grim) (1512-1520) by the prolific Ottoman architect Mimar Sinan, who also strengthened the mosque’s walls with exterior buttresses. The two remaining minarets – which match each other, but neither of the two existing ones – were added during the reign of Murad III (1574-1595). Thus, today the grandeur of the Hagia Sophia is unfortunately surrounded by a hodgepodge of mismatched minarets, none of which (with the possible exception of Sinan’s) seem to fit with the building.[ii]
In conclusion, there is not any significance with regards to the colour of the minaret. The different colour is due to the different material used to construct the minaret. As the minarets were constructed by different people in different times, each minaret was constructed using different matter available in the era.
And Allah Ta’āla Knows Best
Mufti Faraz Ibn Adam al-Mahmudi,