Pigments can be rated according to their transparency or opacity. A good way to test your paints is to spread a thick layer, called a masstone, across a white gessoed surface (far left swatch).
Next to that, spread a thin, transparent film, called an undertone (swatches 2 and 3). A dark mark on the board behind the film of paint helps to test opacity.
The colors in these swatches are ultramarine blue, Venetian Red, and terre verte. The Venetian red is the most opaque of these three.
You can also test the pigment’s tinting strength by adding white. Tinting strength refers to the ability of a pigment to maintain chroma with the addition of white (remaining swatches). Some colors, like pthalo blue are powerful in tints. Some, like terre verte are relatively weak.
Most colors shift a bit in their color temperature. Warm colors typically shift to the cooler or bluer side.