From: http://www.gallipoli1915.org/David Nicolle's Osprey book 'The Ottoman Army 1914-1918' says that the standard Ottoman Army summer/tropical uniform was white for the 100 years up to the beginning of WW1 and that it was still being phased out during the War. He has photos of troops in white at Gallipoli in 1915 and a photo of a medic in Palestine in white with a red crescent arm band.
Some of these Askeri are wearing the tropical uniform?