What is probably indicates is that he has been writing with a quill pen. Fingers get very inky.
gee you think, it's from a quill pen...and the Iraqi people will see this portrait with intense emotion about see the purple finger. We missed the detail, so, we expect that they won't. And, after the word gets out, they will send off for the portrait on e-bay.....
I go with the quill idea and nothing to do with Iraq.......