Question: Artist's Copyright FAQ: What's the Situation with Photos of Famous Paintings?
If a painting is no longer under copyright, a color transparency (and probably a digital scan of that transparency, though some scans have 'watermarks' which are copyrighted) is also not under copyright in the USA according to a 1998 case between the Bridgeman Art Library and Corel Corporation
. (See also this Cornell summary
.) It's important to check whether the original painting is out of copyright. If you paint a copy of an Old Master, it's convention to write "after [artist's name]" so that the viewer knows the painting's origins.
Go to Full Artist's Copyright FAQ.
Disclaimer: The information given here is based on US copyright law and is given for guidance only; you're advised to consult a copyright lawyer on copyright issues.