Looks like you have some choices ?
http://www.fas.org/nuke/guide/russia/airdef/mig-25.htm(scroll to bottom for the pics)
That's the first good pic of a camo one I've seen.
For the "standard" grey overall version The paint reference I found says:
Primary / Overall FS36375 (Gray)
Humbrol 127, Xtracolor 136, PollyS 500836, Gunze H308, or Testors 1728
Darker panels FS36231 (Gray)
Humbrol 140, Tamiya XF-54, Xtracolor 131, PollyS PCA 824, Gunze H57, or Testor 1740