Name two grapes and you have triggered a specific set of rules: name them all, show the percentages, and back it with an appellation.
 
Putting a grape variety on the label — one grape or several — is a regulated claim, not a marketing choice. The rules are mechanical: easy to meet once you know them, easy to fail if you do not. Find out how to label a varietal or a blend so it complies with TTB's COLA regulations. Link to the complete how-to article here.