As with many aircraft, I would say that there's probably going to be some variance. For example, here's a MiG-21 cockpit, looks more green than gray to me:
https://commons.wikimedia.org/wiki/File:Cockpit_of_MiG-21_preserved_%C5%81%C3%B3dz-Lublinek_(3117340809).jpg
But also, there's this example, a MiG-25P, that looks more blue than green to me:
http://www.sas1946.com/main/index.php?topic=37401.0
Scroll down on this one, and you'll see that bluish color again, this is front and back cockpits on a MiG-31:
http://www.ausairpower.net/TE-Foxbat-Foxhound-92.html
Here's a -25 cockpit, with the gray color you indicated above:
https://theweekendmodeller.files.wordpress.com/2015/08/mig-25-ru-4-inside.jpg
If I had to guess, I would say that variances could be from different batches of paint, or whether the a/c had gone through any modernizing programs where they may have changed things in the cockpit, etc etc. maintenance could also be a factor, if there were a need to repaint the cockpit, I suppose. Keep in mind that old Soviet aircraft were not always maintained very well, and so I would guess that some things would not have mattered so much, such as what color the cockpit might be repainted. You might find that any of those colors would be appropriate. Soviet jets are not generally the same as American ones, in that we seem to have kept far more detailed records here in the US about which specific aircraft went through what maintenance or upgrade programs than the USSR did. If you were looking to build an F-14, for example, you could look up the bureau number for the specific jet you are modeling and determine exactly what was done to it.