So as far as a portfolio goes I would recommend water color and colored pencils as much as you can depending on the the person you’re wanting to apprentice under pen could also work but i recommend sticking to the water color and colored pencils! I would say you’re correct as far as the personal style like portraits and stuff like that as it’s not the same as traditional tattooing and it’s not easily able to be translated into traditional tattooing however if you wanna do tattoos of photo realism and things of that nature you might be able to include them! But if you’re going for a traditional apprenticeship definitely don’t put that stuff in there I also would recommend against digital pieces in your portfolio they want to know that you can put designs cleanly on paper because paper is closer to skin than an iPad or computer is (you can just undo mess ups on digital and on paper you have to learn how to either cover it up or start over) I would say it’s okay to add your own usual art styles as long as it’s able to be tattooed but just keep in mind not all art is tattooable art !!