You havn't posted any pics or profiles of the particular aircraft.So it is quite difficult to say what tonality it was.According to the NKAP no.547ss order from 20th June 1941 undersides of planes were painted with AMT-7 Blue-Grey or AII Light Blue. For matal covered surfaces the A-28m oil enamel might have been used.But there was shortage of the enamel at that time.So surfaces painted with the paint were rquite rare.
For the AII ( FS25466 ) colour you can use Humbrol 65, Gunze H67, Testors 2126, Tamiya XF23.
For the AMT-7 ( FS25550 ) or A-28m ( FS35250 ) paints you can use Humbrol 115, Pactra A109.
Unfortunately the Humbrol 115 hasn't been offered yet. But I can recommend another firm named White Ensign that offeres a very nice set of the Russian colours of authentic tonalities. Please check here...
White Ensign Models home page for ship kits and upgrades