In days of old artists would often use something called a Claude glass otherwise known as a black mirror.
The idea was to reflect the scene (it was more for landscapes than portraits) with the colour largely suppressed.
It’s a useful method to see the scene as purely values and not be distracted from the colour. If a painting isn’t working well then it’s fairly often due to incorrect contrasts rather than incorrect colours.
You can do the same these days considerably easier with your phone’s camera. Make sure that any auto enhancing gizmos are switched off and then take the picture in monochrome or modify it afterwards by setting the colour/saturation to zero.
Still a little way to go on this one but it’s coming along…