According to Meteor, Sabre and Mirage (Stewart Wilson), the double grey is real.
Mirage IIIO schemes:
Natural metal (initial scheme)
Green/Grey (As stated above, and in the mod)
Two tone grey (As stated above, and in the mod)
Dark Grey upper, Light Grey lower (not in the mod)
ARDU (Aircraft Research and Development Unit) had some of the best looking Mirages, they had a striking Dayglo and White colour scheme:
http://www.adf-serials.com/gallery/albums/...RF_3A3_115a.jpg
http://www.todo-aviones.com.ar/francia/mirage3/3a3.jpg
Prior to this scheme, they had a yellow-upper, dark green-lower scheme
One more, for good measure
http://www.todo-aviones.com.ar/francia/mirage3/3a3.jpg
For the 60th anniversary of the RAAF's formation, a special Mirage scheme was used, of white with red and blue trim:
http://www.gjsmith.net/images/Textures/Mirages-A3-15.jpg