I prefer XML with a DTD because it can help prevent size=blue or color=large type mistakes...
You've said this multiple times, but you've never explained how.
The how is the same way you would do it for any other type of file. Someone writes a comprehensive set of rules by which to validate the configuration file. The file format doesn't matter. The format of the rules doesn't matter. Something reads and processes the configuration file and the rules and applies the rules to the processed file.
You have to do this anyway to parse the configuration file in your application.
Now you may have better tools to do this with an XML application than with any other format, but then your real argument for using XML is better tools, not validation.