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In other words, the colors set in Preferences > Document Classes > [TYPE] > Colors, like "Current line" and "Keyword 1".

I have changed some of mine and I'd like to back up that file. I looked through the various directories in Program Files/Textpad/ but couldn't find any appropriate files. I'm using Textpad 4 by the way.

They're not in a file; they're in the registry, at
HKCU\Software\Helios\TextPad 5\Document Classes\[class]\Colors