Hi Boris!
Glad that you understood my meanings! 

I agree with some of your inputs, and also would like to add some more - also from analysing pictures, reading and seing some real subs on dry dock - not type VII-C's - I am not that old 

When you make a VII-C and going to weather it , you have to consider the period of action (Paukenschlag or Happy Times versus the Bad period - the Hunted).
So, during Paukenschlag the U-boot force was most of the time allowed to surface at will and only went subsurface when really needed (attacks or being attacked). In this conditions the boat would suffer from "air exposition" with all the inerent rusting and weathering effects (not only rust, but paint fade and flake). Bellow the "waterline" the natural growings would happen as well as the usual corrosion to the paint and metal. A simple bump on the pier or to another boat while on harbour could make "nasty" looking effect to the outer hull...
The repainting would happen mainly to the upper works while on harbour or even sometimes while at sea, being these easily striped and making different tones and patches of the colour used, revealing the under colour. 
Only on a grand repair or after a few cruises, and if it was really necessary, the boat would go to a dry dock and then all the hull got proper atention, making it "new boat". 
After 1943, the conditions changed and the U-boats no longer could carry their "usual procedings", making the boat stay underwater more time, being attacked more often and not even during night they could surface confident to recharge the batteries, so the VII-C (and variants) and the IX-C and D are heavily weathered to all this situations - Salt can make a lot of damage! Schnorkel helped, but the boat would be  underwater most of the time. 
If you have the chance to see pictures of U-505 (a type IX) when it was captured you can see the amount of damage to the boat and paint that I have stated. I am not saying that all the boats would have this kind of look, but I can "guess" that they would look almost like that - depth charging and poor quality of the paints, small periods on harbour (with all the implication that comes with this "pit-stops" instead of major repairs).
Electro type boats (Type XXI and XXIII) suffered a lot in the last months of war. Try to find a Adalbert Schnee Type XXI picture and you will understand 

What do you think?
Skipper