What is the monochrome-mode in drawing ?
If you want color in your drawings, you may want to switch back and forth to make sure things are displayed correctly when you plot them.
I added a button to quickly switch back and forth between the 2. Though personally I mute all of my backgrounds to grey in the software and dull them with the monitors. bright white hurts my eyes after awhile
I like a black background. But, I use monochrome when using the track drawing changes "overlay" command. This way I can really see the differences in the two drawings.
"So if I select this monochrome selection what happens ? Does it change the files somehow ? What harm it causes if any?"
You can turn the monochrome option on/off at will, it will not harm your file in any way. It is only a display option. It may mark the file as "modified" because you have changed a display option, but the underlying object colors are retained. Your drawing may plot differently if you use the "colors: as displayed" option.
You can also think about it this way. The color option maybe nice for doing your detail work because you can see the different parts in your assembly. After that you turn on monochorme to see what it may look like if you do not have a color plotter.