Black-and-white monitors are preferred for obvious reasons...

the ST's 640x200 resolution in colour mode leaves much to be desired, so when a black-and-white monitor isn't available on your ST, the Spectre software gives you the option of doing some creative dithering to cram an entire 512x342 Mac screen into the same number of pixels.

Another option is to view just half the Mac screen at a time, pictured next...