It is always useful to test the properties of a paint whenever you buy a new one so as to make sure you don`t get a nasty surprise when you are painting. Transparency of a paint becomes a very important property when you are using glazing. A transparent paint layer will allow the previous layer that was painted to shine through, modifying the overall color. This is useful when you want to make subtle changes in color. If the paint is completely opaque, you cannot use this method of glazing.
The labels for colors stating their properties are sometimes ambigous across different manufacturers, so to make your own transparency chart, draw a vertical strip of line with a waterproof black marker on a watercolor paper and paint bands of different colors across it. The ones which disappear completely while crossing the black strip are transparent, the ones completely obscuring the black strip are opaque and those that form a slight film over the black strip lie somewhere between transparent and opaque depending upon the thickness of the film. For e.g., WN winsor orange is quite opaque, it covers the black marker line; S translucent orange is very transparent. MB burnt umber is transparent, DS Lunar earth, even though it is of a lighter value, covers the black line almost completely.
Another property that can be tested alongside is the flaring or spreading quality of a paint. Wet an area of paper and touch the tip of the brush loaded with the color you want to test. Some paints flare very quickly on the wet paper and some tend to settle somewhat whereever you put them. For e.g, MB burnt sienna does not spread very quickly, whereas S walnut brown does. When you mix such type of colors, they might separate out when applied on the paper. In the swatch below, you can see the burnt sienna separating out at the fringes, even though they were mixed thoroughly on the palette. You might want such effects or they might be undesirable sometimes, so its good to have this kind of knowledge beforehand. When you want to save a highlight, for e.g., a flaring color might spread too rapidly into the area that you want to keep white. In large, background wet in wet washes too, it is useful to know of this property.
Granulating or sedimentary property is a unique quality of watercolor paints. To test this, apply a fluid paint solution over a wet area in a single stroke. Rock the paper to and fro if required but donot fiddle with the brush, else you will ruin the effect. Some colors like the pthalos will have a smooth appearance, some like ultramarine blue, cobalt blue, a few earth colors will have a mottled appearance created by pigment particles settling in the tiny valleys of the surface of the paper. Non-granulating paints are desirable for painting flowers, leaves or other objects having a smooth appearance. When painting sand, rusty hinges, cement walls etc., granulating paints will give a textured look without the need for any laboured brush strokes. This swatch is of DS blue apatite genuine, it shows a large-grained texture. The one below this is of DS ultramarine blue, it shows a very fine, powdery texture.