Values. It is all about values.
IMHO 'greyscale' and 'b&w" are the same thing.
If you have Photoshop or a similar program on your computer, it is easy to change a photo of your work to B&W and check if the values are OK with you.
original phot (a ref for a painting)
cropped for composition, changed to B&W (aka greyscale) with very high contrast to ID values
this is what I do to help me 'see' if the phots/scene will work as a painting
I often just squint at my work as I go along from a distance of at least 3 feet to see if has proper values. Or look through tinted glass to convert it to a monochrome.
All this is explained in much more detail in almost every "how to paint' book I own. Color theory classes and tutorials also explain how to do this better than I could do.
Buy as many books as you can, read them and study them, doing all the practices they suggest. and if there are any questions about what you have read, ask them here, we are always willing to help you learn.
life is good