I'm sorry to destroy your precious recent efforts but I tested the game with 16-bit colours and there was no improvement :(
Here is what I got from the support. I'll do it "later" after sleeping and working.
Currently our German support is overwhelmed with work. If you don't mind I will reply using English language.
Please try following these troubleshooting steps:
1) Please make sure that you use "Run as Administrator" to install and to start the game (right-click on the shortcut or setup file and select "Run as Administrator"). If you are using Galaxy, then make sure to run Galaxy as Administrator.
2) Please reinstall the game AND add it to your anti-virus/firewall software's exception/trusted list. Simply disabling firewall or AV software could not be enough, since some of these software are running still in background or in services list. If you are using Galaxy, then please add Galaxy as well.
3) Reinstall the June 2010 DirectX runtime package. You can find the archive file containing the 2010 package installation files here: http://www.microsoft.com/en-us/download/details.aspx?id=8109
You will have to extract the files into some folder, then open that folder and launch the DXSETUP.exe file in order to proceed with the installation.
After that please download and install DirectX Web Installer: https://www.microsoft.com/en-gb/download/details.aspx?id=35&e6b34bbe-475b-1abd-2c51-b5034bcdd6d2
Please have in mind that this will only add DirectX 9.0c to available pool of DirectX libraries, and it will not overwrite your other versions. Some games are not able to utilize newer DirectX libraries, and they require DirectX 9.0c.
4) Please update your graphics device drivers: http://www.nvidia.com/Download/index.aspx
5) Please try to installing compatibility mod from PC Gaming Wiki: http://community.pcgamingwiki.com/files/file/699-simcity-3000-compatibility-fixes
Please copy archive's content to game installation folder\Apps (default: C:\GOG Games\SimCity 3000 Unlimited\Apps) and run Install.bat.
6) If above didn't help, then please go to game installation folder (default: C:\GOG Games\SimCity 3000 Unlimited\). once in there find "Launch SimCity 3000 Unlimited" shortcut right-click on it, and select Properties. In Properties window, go to Shortcut tab and in target field add:
If you installed the game to default location, then it should look something like this:
"C:\GOG Games\SimCity 3000 Unlimited\Apps\SC3U.exe" -w -r:800x600
Press Ok to accept changes. After that run the game using modified shortcut. The game should start in windowed mode. Once in Main Menu, go to Preferences -> Screen Resolution -> and here select 1280x1024 resolution. Accept changes to restart the game, and game should start in higher resolution.
Customer Support Representative