In mixing Pantone colors we encounter the phenomenon of “transparent white”, which in other fields would be called “clear.” Many have found that for letterpress printing, opaque white works better for color consistency or for other reasons. I’ve found that with transparent white it can be tricky to maintain even color over a run, as slight variations in the ink film thickness show much more than with opaque ink.
Since the PMS formulas don’t use opaque white, some adjustments are sometimes required. A pastel color may specify 97 parts trans white to 3 reflex blue, for example. Transparent white ink is actually somewhat yellowish, as it contains linseed oil, resins, and other things essential to being ink. So, often we need to add some yellow into the mix when using opaque white.
Replacing transparent with opaque white is also essential when printing over darker papers. If the artwork will allow it, the piece can be run through the press two or even three times to achieve fairly complete opacity. On a hand-fed press this can be quite a stunt for testing consistency of registration, where on a windmill (for example) you will rarely see a variation if everything’s set right.