To be more specific: This is not one of the A310-304 for VIPs or governmental flights, this is one of the four MRTT (Multi Role Transport and Tanker) or the PAX Airbus. Only these A310 are painted grey.
The official governmental / VIP aircraft (registration number 10+21 Konrad Adenauer) is painted white, also like its backup aircraft (10+22 Theodor Heuss). The PAX Airbus (10+23 Kurt Schumacher), sometimes used for governmental flights, is painted grey.
10+24 Otto Lilienthal, 10+25 Hermann Köhl, 10+26 Hans Grade and 10+27 August Euler are the MRTT, used for PAX, freight, MedEvac and as air-to-air-refueling, are grey.