Leave it to the Swedes to have the best cross reference on aircraft paint schemes. Try this link to the soviet page, they even have color chips (of course they are limited by capabilities of you monitor,etc. and can not be relied on). The site says the gray on a Su-24 is FS 36375 it lists the number for Humbrol as 127, Model Master as 1728 and other paints but not Tamiya.
www.ipmsstockholm.org/colorcharts/stuff_eng_colorcharts_soviet.htm
I believe this link answers you color question. Good Luck on your Fencer