TRANSPARENT VS. OPAQUE WHITE INK Edit
Most Pantone mixing formulas call for use of transparent white ink, but many letterpress printers choose to use opaque instead. One reason to use opaque white is that the mixed color—off press—may resemble more closely the printed color, though the mixed ink will not be perfectly opaque once printed. When printing on an off-white paper stock, the opaque white helps the ink color to retain its hue instead of allowing the color of the paper to show through. Use transparent white if you intend to do overprinting to mix colors (ex: yellow and blue to make green), or if the printed image is meant to be translucent. It is always advised to test your ink on the stock you’re going to print on before inking up the press, either with by “drawing” the ink down with your ink knife, or by making a thin smear with a finger.