How can I contact the author or uploader of a paint in the Showroom?

When viewing a paint in the Showroom, there are details about the author listed at the bottom of the submission page.

If the user allows messages, a message icon (envelope) will appear next to the user’s name if you are logged in.

You could also contact a user through the iRacing messaging system. Names and Customer ID numbers on Trading Paints match those on iRacing.