Fix Rainbow Colors

1. Run your game at least once and exit out of it.
2. Go to your registry and navigate to "HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\DirectDraw\MostRecentApplication" (if you're using 64 bit, go to "HKEY_LOCAL_MACHINE\SOFTWARE\Wow6432Node\Microsoft\DirectDraw\MostRecentApplication")
3. Take a note of the name, game.exe, and the ID, which looks similar to this: 0x02e41c67
4. Remove the '0x' part of the ID and separate the values into pairs, like this: 02 e4 1c 67
5. Take the pairs and reverse their order, like this: 67 1c e4 02
6. Open notepad and add this:

32 bit only
Windows Registry Editor Version 5.00

[HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\DirectDraw\Compatibility\<NameOfGame>]
"Flags"=hex:00,08,00,00
"ID"=hex:00,00,00,00
"Name"="game.exe"
or

64 bit only
Windows Registry Editor Version 5.00

[HKEY_LOCAL_MACHINE\SOFTWARE\Wow6432Node\Microsoft\DirectDraw\Compatibility\<NameOfGame>]
"Flags"=hex:00,08,00,00
"ID"=hex:00,00,00,00
"Name"="game.exe"
*Note: Change <NameOfGame> to whatever you want, leave 'Flags' alone, enter your own ID keeping commas intact, and change 'game.exe' to whatever it is.

7. Save it as 'fix.reg' and run it to import it to the registry.

Here's my example from 'The Hell' mod for Diablo:
Windows Registry Editor Version 5.00

[HKEY_LOCAL_MACHINE\SOFTWARE\Wow6432Node\Microsoft\DirectDraw\Compatibility\The Hell]
"Flags"=hex:00,08,00,00
"ID"=hex:8b,c1,c3,34
"Name"="TH.exe"