As per the Turkish government guidelines, (emphasis mine)
India: Diplomatic passport holders are exempt from visa for their
travels to Turkey up to 90 days. Ordinary, Special and Service
passport holders are required to have visa to enter Turkey. Ordinary,
Special and Service passport holders with a valid Schengen members or
USA, UK, Ireland visa or residence permit may get their single entry
e-Visas valid for one month via the website www.evisa.gov.tr, provided
that they meet certain conditions.
The fact that you are employed by UN and returning from a mission in a third country does not seem to entail special provisions (per the website).
So as per the rules it looks like you need a visa to enter Turkey. Further as (I assume) you don't have a valid Schengen Visa or US Visa you are not eligible for e-visa and must apply at the consulate.
I am not sure whether Indian official passports are treated as "special" or "service" passports, but trying to enter passport details on Turkish e-visa website has the following results:
- For "service" passports (which I assume an official passport should be), there is no visa fee (gratis) but the conditions remain the same (You must have a valid Schengen visa...)
- For "special" passports, you cannot apply for an e-visa and must apply at the consulate.
Since holders of official passports are mostly exempt from paying visa fee and have a lot less documents to submit, it should be straight-forward to obtain a visa from the consulate but there are two major hindrances that may happen:
Turkish consulate may insist on a visa note from your department/ministry of external affairs(India) for granting a visa. I know some other consulates want that (E.g. Switzerland)
The bureaucrats at your parent department may not like it when you hand back your official passport with a Turkish visa/entry/exit stamp. An official passport is handed over to you for the explicit purpose of your mission in the countries you need to travel to. As such visiting a third country may invite curiosity at best and some official show-cause notice at worst (You did not have political clearance to visit Turkey etc.). Maybe I am being paranoid and if you know of people in your department who have done so, then it should be OK.
If you don't wish to "enter" Turkey and are merely transiting Istanbul(Ataturk Airport) then it should be fine provided you are flying with Turkish all the way.