There are some neat tricks you can use to predict how your paint is going to react when you use it. Will it have covering power? Will it be transparent?
How To Tell if a Paint is Translucent or Opaque…Just by looking at it!
Some manufacturers try to list information regarding the physical properties of their paint contained in each tube. The pigment information, for example: pigment py3, pw6 is essential and I would not purchase any paint (for serious artwork) without this on the label. Any reputable brand will always report the specific pigments that are used in the paint so serious consumers can make informed decisions. Some companies even attempt to describe the paint’s color in terms of its hue/saturation/value, (usually through the munsell color system) and its transparency. I find the transparency information to be limiting, and many brands don’t list it anyway so I will teach you how to tell if your paint is opaque or translucent with a simple test.
If your paint looks darker when used thicker it is a translucent. The bigger the change in color value when viewing a thicker layer versus a thinner layer the more transparent the paint is. On your palette, when you have your paint squeezed out of the tube into a large clump, this will be your “thick layer of paint”. Now take your finger, palette knife, or brush and drag a small amount of that paint out from the clump. Make sure it is a thin layer. If both the thin layer and the thick layer (the clump) look the same, that paint is opaque. If not, if there is a value change and the color looks different, the paint has some degree of translucency.
The reason this works is a result of how light rays interact with each type of paint mixture. In an opaque paint the light rays don’t penetrate the paint film and merely reflect off the surface of the paint. So a 1 inch layer of paint looks no different than 1/100th of an inch when using opaque paint. In the case of a translucent layer of paint, the light rays penetrate the paint layer much further, often not stopping until they hit an opaque base layer underneath such as a white ground in which they then reflect back out. So when a translucent paint is used in a thick layer, many of the rays are entering deep into the paint film and getting trapped. If these rays never make it back out we have nothing to see, or less to see…hence the darker color!
Zero light rays = darkness.
This is why thicker layers of translucent paints look darker. This is also the reason a vivid looking glaze mixture on a white palette seems to have no effect when painted over darker areas of a painting. Check out the diagram. Cadmium red medium is obviously an opaque color; note how it looks exactly the same in the large clump or when a thin layer. Pthalo blue and ultramarine blue are definitely translucent colors. Pthalo practically looks black when in a large enough clump. Many light rays are getting trapped in there. Cobalt blue is one of our more opaque blues, note how it changes only slightly when viewed as a thick versus thin layer of paint.