@Lockenheim's advice is good 
what i personally like to do is add a layer of just black on top of everything and then set it to Color and turn it off and on when i wanna see grayscale or not 
kinda like this:
I think your values are lookin pretty good though you may want to smooth things out just a tad bit like around the nose for instance - and also overall maybe use a soft brush on a large size - opacity based on pen pressure and darken the lower edges, left side and lighten up the top a little more.
