As the painting nears completion, I like to do a value check to make sure the values help to support the subject of the portrait.
Luckily, this step is made insanely easy by modern technology. Any iphone or Mac can easily turn a photo into a black and white image so you can check your value.
Once everything is set into place as fair of the larger planes, the success of the painting depends upon the slight adjustments. Which is why checking the value is so important.
I consider myself actually lucky to be colorblind because my world is faded, so I follow value more naturally. of course, it's a double-edged sword because I need pay really close attention to my color palette.