From memory ( not always the best!) it's somewhere near Humbrol # 102 matt army green, sorry I can't find an FS number. As to why, maybe all aviation wheels were produced in the same factory. Or' given Soviet thinking, it's always worked before, why change it?. Have you seen the colour they use on instrument panels? That is also used on most of their aircraft, it's horrible, much more glaring than light grey or black. Why use that one? I have no idea!
Pete