I am still busy working on this. I am not a good painter and am kind of stuck because I don't know how else to give this flat painting some form. I know I should add tone and detail, but I have not established enough technique to execute the application successfully, would love your help. :). Thanks!
A great expression, well done. I think you've got the basic idea, that from is described with values. It may be a matter of confidence. My teacher used to encourage me not to be afraid of using those darks. I would encourage to keep on with practise.
Here's another try. Use soft and hard edges. Soft edges push back and hard edges pull forwards. Warm colours pull forwarda nd cool back. So, the warm colour in your background may npot be too useful. I would use an overall darker background in this case, I suggest you look at the work of some great portrait painters to learn. You could try the BP National Portrait Gallery competition for good examples.
I like the expression on Mandela's face - I think you captured that really well and it's certainly obvious who the portrait is even if you hadn't listed his name in the ref link. One thing that's caught really well, too, is the coloring of the mouth. It is a very African mouth (I draw and paint a lot of people of color) and lots of artists, whatever their race or background, don't get the colors right.
Mike gives you great advice in both of his comments! Keep at this and, if you have the time and the focus, try the technique I mentioned in another discussion - draw or paint this same photo ten times in a row. It's an amazingly fruitful thing to try. Even 5 times would be hugely instructive.