I can think of several examples of chassis numbers in various states of production off the top of my head, I'll see what I can find for photos. There may be decals, but I don't know which set/s they are... Try looking at Archer for starts.... Models I've seen built in this fashion have had hand-painted numbers.
Here is a Maus chassis;
Jagdtiger;
Hetzer;
Elefant;
These numbers appear to be painted. Chalked notes, location / alignment marks and numbers from various stages of production are common too.
Feel free to wade into the whole Lead Oxide Red vs Zinc Phosphate Grey and/or Black argument in order to determine which color you are looking at.....
Most of the hulls you see are in Red Oxide primer, but not always.
An introduction into this quagmire;
http://www.network54.com/Forum/571595/message/1223453004/Re-+s.H.Pz.Jg.Abt+559+Jagdpanther+insignia
Quoting from the final paragraph of the above page;
"It should be noted there are plenty of wartime photos and surviving relic evidence of red oxide/red lead tinted primer coatings which clearly establish it as the most commonly used primer coating colour on Panzer componentry. Modellers in particular should note, however, there is arguably room for some variation in primer colours on their builds of late war vehicles."
The chassis numbers, turret numbers and other component's serial numbers are also stamped into the metal itself, a quick search will show examples of these. Obviously, these would be found on interior surfaces.
I imagine the whole numbering / marking process probably starts with a welder's soapstone marker and progresses from there. Some chassis numbers were even visible on the finished vehicle, many Jagdtigers displayed them on their glacis;
Mig Jimenez's take on this idea;
http://www.missing-lynx.com/gallery/german/migjag.htm