Hello Jess,
Yes, I would say the light grey on U 253 was Hellgrau 50. Since the KM used paints, which varied to quite a degree in colour, there isn't just one FS code for Hellgrau 50. The light grey looks particularly light in the colour photos, so it might be prudent to use the lighter end of the spectrum: Colourcoats' Baltic light grey (an alternative Hellgrau 50) KM13 (RAL7038, FS35492). Or simply a light grey in this area.
There is a much better version of the colour photo of U 253 which adorns the cover of Warship Pictorial 27. This can be found in the Anstriche book, and I can scan and email it you if you like. The light grey looks light grey rather than off-white. Of interest in this photo is that the wooden seats on the deck railings are wooden in colour. Also, the entire fairing in front of the conning tower looks like Dunkelgrau 51. So does the 88mm.
And yes, I would agree that the dark grey on the upper hull is RAL7016, just like the lower hull. The Revell instructions appear accurate in this respect..
Best regards,
Dougie
PS. Jeff, my email address is dougie@martindale47.freeserve.co.uk