Whether a color is opaque (covers up what's beneath it) or transparent is of greatest importance to painters who work with glazes to build up color, rather than mixing on a palette. Not very many manufacturers provide this information on the paint tube label, so it's something you have to learn and remember (see: Testing Opacity/Transparency).
A few paint manufacturers indicate whether a color is opaque, transparent, or semi-transparent on the tube. The acrylic paint manufacturer Golden makes it easy to judge how opaque or transparent a color is by having a swatch of the color painted on the label over a series of printed black bars. The swatch also enables you to judge the final dried color, rather than having to rely on a printed version of the color. If you notice some variation in the swatches between tubes, this is because they're painted by hand, not by machine.
