Dark skin tone


Mac-Valve Maestro!
So i’ve decided to work the portrait for my friend and its obviously my first attempt at a full colour piece.
So i’ve done a lot of research and lerned I need to add a base tone down for the lighter shades, then add the mid tone layer and then onto the dark tones.
The portrait is of an African lady with a really dark almost drak chocolate brown tone.
So i’ve been practising today to get the tone down and have found dark umber would be the best place to start, so a few questions.
If I want to lighten the tone once I get there I just add white to the mix?
To darken dark umber cobalt blue or ultramarine blue would be the best bet?

So am I to colour match the skin tone the best I can say for the mid tones then lighten this for the lighter tones and darken for the darker tones or shadow areas?
I’m having pretty bad trouble at the moment colour matching but i’ve come close a few times.
I’m using CI paint which are transparent so not sure mixing in a pot and adding to the brush is the correct procedure but i’ll perservere for now.
Any help would be appreciated.
Here comes another lesson. I love it when I can learn with you guys, needless to say I write all of this down in my mixing suggestions book that I started specifically for this. I will be following this thread Jimmy, good question, but I thought Mitch explained to use the color that is predominant, then darken the shades and tones? So perhaps to get the base color first then only darken rather than having to lighten again?
I got roughly there in the end with much mucking around.
I tried to start by using dark umber but in the end made it using primaries.
I mixed the same amount of each primary and added white that I read to do online but got grey so rather than start again I added a healthy dose of yellow and red, I just kept adding red as you could see I was getting a nice shade of brown.
Then I added small ammounts of blue which darkened the mix. I found it looked to have too much of a cherry tint when I sprayed so I added yellow and ended up with a nice golden brown colour at light opacity but you get a real nice dark chocolate colour when sprayed heavier so it will be the tone I stay with.
Got there in the end which was nice, its not perfect copared to the reference but I’m not really going for photo realism as its not my level yet so making a good painting that does it justice i’ll be happy.
Hope that helps you Izle