Whether you're painting with watercolors, acrylics or oils, it's very important to understand the opacity and transparency of your pigments. All art media are made of pigments suspended in some kind of medium, whether it's gum arabic as with watercolor, liquid plastic as with acrylics or linseed oil as with oil paints. The medium affects the transparency and opacity to some extent, but the properties of the ground pigment are also important.
The easiest way to begin to understand the differences between opaque and transparent paints is to experiment with watercolor. Watercolor paints are classified by some manufacturers as transparent, semi-transparent and opaque. Transparent colors, when brushed over other layers, allow the layer underneath to shine through, whereas opaque colors tend to cover up under layers. But all watercolor paints are somewhat transparent; that's the beauty of the medium. So Caroline Buchanan, who has been teaching watercolor classes for years, prefers to categorize them as staining, sedimentary, and luminous. The staining colors are like transparent dyes that cannot be easily lifted from the paper once they are dry; the sedimentary (or opaque) colors can be washed over the staining colors for subtle, granular effects; and finally, the luminous colors are the most transparent and easily lifted, and they sit on top of all the other layers. This method prevents what Buchanan calls "mud," or the unintentional mixing of layers.
Many watercolor artists, while they love the transparency of their medium, occasionally long to be able to "cover up" an underlying layer. Especially in the 19th and early 20th century, watercolor artists like Winslow Homer mixed Chinese white, an opaque color, with other colors to create "body color," or a more opaque flesh tone that could be used to insert a human figure, for example, into a landscape painting by simply painting the figure in gouache over the transparent watercolor landscape. Nowadays artists can buy gouache paint in tubes in many colors. It can be an opaque medium on its own, or it can be used with transparent watercolor to add more opacity to a painting.
Acrylic paints are somewhat more opaque than watercolor, but each color has its own properties and relative transparency. Some paint manufacturers, such as Daniel Smith, designate the opacity of each color in their catalogs. As with watercolor, the more transparent colors such as the quinacridones can be used as glazes over the more opaque colors such as Indian Red and Chromium Oxide. Any color mixed with opaque Titanium White will look more opaque and will cover other colors.
Oil paints have some of the most gorgeous transparency qualities of any art medium. Artists have been using these qualities for centuries to create layers of luminous glazes. The advantage of using diluted, transparent colors in oil painting is that you can create an underpainting, or "grisaille," with, say, Sepia, on the raw canvas that establishes the values early on. Let this layer dry thoroughly, then brush on layers of transparent colors such as the Quinacridones, diluted with medium. You can add bits of semi-transparent or opaque paints at the end. If you are not sure if a color is opaque or transparent, use a permanent marker to make a line on paper or canvas. Paint over the line with the color. If the paint covers up the line and you can't see it, it's opaque; if you can see the line, it's transparent.