Calling out to all of you MIG experts!

I stumbled upon a very attractive image of an abandoned Romanian MIG-15 UTI: http://www.airplane-pictures.net/photo/176549/2579-romania-air-force-mikoyan-gurevich-mig-15-uti/
As far as I know, these were all wearing natural metal finishes so I was wondering whether this information is wrong or if the gull gray tone to be seen in the image is oxide that accumulated with time and has been peeled off where the lighter tones shows...
Any ideas?
Many thanks in advance!
Cristian