I've been using two monitors since the late 1980s, when it became possible on Macs. I've usually had one very big one for stuff like music pages and a smaller one for the Finder and other user interface controls. Both were greyscale back when I started. I've never had any problems with the OS, on any MacOS from System 7 on.

I rather miss monochrome. Size and resolution are more important to me than colour.