The German feldgrau field uniform was introduced in the decade prior to WWI. Experiments in clothing soldiers in more subdued colours took place in 1907-1908 and, being judged successful, led to the general introduction of the feldgrau uniform in 1910.

From 1910 feldgrau was worn by all ranks for field service, and enlisted men also wore it for fatigue duties. The "blue" uniform continued to be worn on all other occasions (I put "blue" in scare-quotes because of course many regiments of the German army wore different colours, although dark blue was universal for Prussian line infantry and artillery). This state of affairs continued until a little after the outbreak of WWI, when new uniform regulations were issued. From the 1st August 1914, feldgrau was prescribed for all orders of dress except parade and ceremonial uniform, restricting the "blue" uniform to the most formal occasions.

The next major set of uniform regulations came on the 21st September 1915, and these abolished the "blue" uniform completely, except for the guards heavy cavalry and some hussar regiments (unfortunately I cannot find which specific ones), who continued to wear their old uniforms on the occasions described above. The pre-war uniform of the heavy cavalry was white, so unless any blue-coated hussars were exempt from the 1915 regulations this would be the date the blue uniform officially disappeared.

To compensate, the 1915 regulations also introduced a new uniform for parade, ceremonial and walking-out occasions. This new uniform preserved many of the tailoring features of the "blue" uniform, but was grey in colour (with collar, cuffs and piping in various colours depending on the unit). This uniform was never issued on any large scale and most soldiers wishing to obtain one had to purchase it themselves. This remained the regulation up to the end of the war.

Now, the above refers only to the regular army: while the feldgrau uniform seems to have been efficiently rolled out to the first-line component of the army in 1910, the reserves, the Landwehr and the Landsturm had to wait longer to receive it: in some cases this took until 1916. Units which hadn't yet been issued the feldgrau uniform continued, faute de mieux, to wear the "blue" uniform.

Nigel Thomas, The German Army in World War I, 1914-1915 (2003)

Stephen Bull, World War I German Army (2001)

DSV Fosten & RJ Marrion, The German Army 1914-1918 (1978)