I wonder if the designers of cold war soviet planes read the same color theory because their cockpits are always a very particular indescribable shade of green. There were also very specific colors for subsystems, yellow for fuel, purple for hydraulics etc. Much more than the contemporary US designs.
That color shows up a lot in stairways apartment blocks and school corridors and bathrooms in ex-Soviet bloc countries.
My two guesses are that it was colored like that get the pilots feel like they were in a particular environment - a familar but not exactly private or comfortable one. It's a cultural thing like if you paint a bus yellow, Americans will think of a school bus, but most other people won't.
My other guess is that they only made certain kinds of dye, and its very well possible the same factory made it that made it for bathroom tiles. In capitalism, if you don't have orange paint, for example, some company will just start making it if there's a demand.
In communism, if nobody makes it, then it's not available, until and if some comittee decides that it should be made.
My father was a mechanic and crew chief working on F-14's during his time in the air force. His two takeaways from his service were: 1. No one should ever join the military for any reason ever forever, and 2. Somebody needs to color code literally anything.
He talked about how the wiring schematics were a maze, made worse by using only non-labeled gray and black wires with connections and mounts that were the same color made of the same material.
The exterior being gray makes sense - harder to see with human eyes. But internals? They should be massively contrasting colors for every single series of pieces to be removed so you can just follow along by color.