There are both. Generally the yellow scopas varient are more common in the trade. These though do not show black on the body but more of a muddy brown color when stressed generally. The ones I have seen in person 'happy' colors are bright yellow with the scopas tang patterning.
I have seen pics of a yellow tang with a black vertical stripe downs its body. I have also seen pics of abberant yellow tangs that show white and black blotches. All of these are very rare and I have not had the opportunity to see any of these in person.