I know that verjus (or verjuice- sour juice from unripe grapes) was originally used for Dijon mustard, and was replaced by wine as it became more readily accessible. However, I can't find much information on the types of wine one might use. I'm sure something comparably acidic would be ideal, and I haven't heard of anyone using red wine. Assuming I'm right, this still leaves a fairly large range from the relatively common Sauvignon Blanc to more esoteric varieties like Colombard and Kerner. Is there a standard wine, or some rule of thumb beyond matching personal taste?