What medium makes the tattoo flash sheets?

I have seen some artists with flawless looking flash sheets, and others that look a little lousy and I think it has a lot to do with the paper and medium they are making them in (and the skill of the artist of course...but If you have looked at a lot of original flash I am sure you know what I am talking about)

So, how do I make some killer flash sheets? I have the talent. I need to know what supplies to use! Some people say watercolor...but I don't think that is what it is that looks crisp.

Thanks in advance!