Sofa green colour with light black effect

Dear Sirs

I am trying to renovate a green leather sofa which has a vintage effect made with black (see picture). I wonder if there is any chance to reproduce the same effect. I guess I should follow your tutorial to repair the deep cuts and scratches and then use correct RAL green color but after this how can I reproduce the light black effects? I suppose to buy a small bottle of black color but I need to dilute it. How can I dilute it? which is the diluting agent to be used?

Thank You 

Kindest regards