When a film is colorized it is regarded as a new work, and is copyrighted
that way.  If a film is up for renewal, then all the studio needs to do is to
renew it.
Regarding (was it Carl Pryluck?) the idea that one can copyright a work that
is in the public domain:  this is not true.  Once a work enters the public
domain, the copyright laws mandates that it stays there.  It is not possible
to copyright a work in the public domain.  So what many companies do in order
to obtain a copyright on a public domain film is alter it in some way.
For instance, INTOLERANCE entered the public domain last year.  But the
recently "restored" version is able to qualify as a new work, since the
copyright falls on the reconstruction.  (Sometimes videocassettes specify
these;  I've seen a tape where there were several different copyrights noted,
including packaging.)
It's a tricky business, and I don't claim to be an expert (even lawyers often
have problems with these things on occasion).
Bob Kosovsky
Graduate Center -- Ph.D. Program in Music(student)/ City University of New York
New York Public Library -- Music Division
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Disclaimer:  My opinions do not necessarily represent those of my institutions.