Thank you for the replies I received on this. I am presently unable to
say what the source was because the problem has gone away again before I
could fix it. Its been a day now with perfect color again.
Intermittent problems can drive you nuts!
The cable seemed a reasonable explanation so I disconnected it and found
no bent pins, but the connector was full of dust (somehow I hadn't noted
this when moving it between computers). I thought that might generate a
short but the problem was not immediately fixed upon reconnection, I
simply found it working again a few hours later.