The professionals in the men's swim brief business have a rating system in place for the amount of transparency shown through different types of fabric material.
They call it 'sheer rating'.
WHAT IS SHEER RATING?
" It means how see-through the garment becomes when wet. 1 on the scale is the least see-through, 10 is the most see-through. As an example, Black is not very transparent and has a lower sheer rating, whereas White becomes highly transparent when wet, therefore has a sheer rating of 10 on our scale. "
THE LINK:
http://www.aussiebum.com/faq
(THIRD TO LAST QUESTION UNDER FAQ's)