The allure of the rhino blanks for me has been they are my goto blank when I’m in a hurry. I haven’t yet had to back paint the blank before glueing up saving me a day waiting on paint to dry. I also love the look of the blanks.
I paint both no matter how opaque the color appears. The other thing I am mindful of is seeing the painted drill/rifling marks. For me, some colors/hues just are not right for the look I want to achieve - remember, I said, "For me."
To paint or not also depends on the pen itself. A Sierra style pen has very little material left at the tip, so the tube tends to show if the blank is at all transparent. Some of the Sierra variants, however, leave more material (LeRoi V2 comes to mind) and will be more forgiving. But of course, when in doubt paint cuz a tube that shows through looks awful.
Cut a thin slice off the end. Smooth and polish it first if you like. Take the slice and put it on something brassy like a trophy and see if it shows through. If it doesn't you don't need to paint it. If it does then put it on various colours to find one that you like and use that colour.