Jasen Betts wrote in a message to Roy J. Tellason:

 RJT> Anybody run into this one before?  I suppose the next step is to
 RJT> see if the problem resides in the monitor or the video card.

 JB> video card.

So it appears,  as I found when I switched the monitor cables and got the
same result on a different screen.

 JB> sounds like the monochrome detect pin may be dirty and/or the card
 JB> faulty.

I suspect that it's faulty,  as the card was acting strangely before this
happened,  I was intermittently losing the blue color,  which also looked
pretty weird.  Now I've been seeing monochrome for a number of days now, 
and I'm really getting tired of it.  :-)  Time to swap out the card,  stick
an old ISA card in there for now.  Which ought to be plenty for this
machine anyhow...

---