It's always a good idea to contact the designer and ask for permission. Most designers out there are very friendly and normally don't mind if you repaint their aircraft, as long as you inform them first
Attaching a screenshot is always another good thing to do. That way the designer can see what you plan to upload before it is uploaded.
Regards, Stratobat
'If the literal sense makes good sense, seek no other sense lest you come up with nonsense'
Another suggestion - rather than release the complete model (again) - presuming with the devloper's permission - a repaint can be released as " textures only", with the appropriate additional aircraft.cfg entries as a text file.
That way, a model doesn't have to be re-downloaded.
and following on from felix also inform the downloader of where to get the base model like a URL or something and state clearly that it is textures only.. fustrating when it says something like "you need to get bill bobs cessna first " with no indication of where to get it from