A small bar that can be positioned around the edge of the screen. I use the Goodnotes app to take notes during Uni lectures and I find myself using the brush pen for titles and key info, and ball pen for the notes. When you have the app on half screen so that the powerpoint takes up one side you lose the access to the quick "one tap" colour selection.

The ideal quick bar would be able to move to either sides of the document you are taking notes on. You would be able to had a set style and thickness of pens within this as well as highlighters. Therefore during a note-taking session all it requires is one click to use a certain pen for the title and one click back.