The power of staining watercolor paint is tied to the pigment used to make the paint, and has nothing to do with the transparency, lightfastness or opacity of the particular paint color.
Instead, the staining power is a measure of how strongly the paint pigment particles or granules embed themselves into watercolor paper. This can be influenced to a lesser degree by the type of watercolor paper on which the paint is used.
What all staining watercolor paints have in common is that their pigment granules are physically tiny when compared to the pigment granules of other non-staining colors.
These tiny granules are small enough to embed themselves in watercolor paper fibers, especially if the paper is very porous. Once a staining paint is dry on watercolor paper, it is almost impossible to remove it without damaging it.
effect is in contrast to “granulating” colors which have large pigment
particles (granules) that tend to sit on top of the paper surface. Because granulating
paints sit on top of the paper, it’s much easier to come back with a wet brush
and remove them after they dry.
Note that the power of staining watercolor goes for most other porous surfaces like your plastic palette, and your hands, clothes and towels. If you get a staining color on a porous surface, good luck getting it out.
Paint manufacturers designate the granularity of paint colors in different ways. Schmincke Horadam colors are marked as staining, semi-staining and non-staining.
Holbein uses the terms non-staining, semi-staining and staining.
Winsor Newton gives a “granulating/staining”
rating, and states that in general terms, their newer modern pigments (Winsor
colors) tend to be staining because the granules are finer.
Daniel Smith (DS) rates them as either non-staining, low, medium or high-staining. Daniel Smith staining colors include Permanent Violet, Cerulean Blue, Cadmium Red, Cadmium Yellow, Azo Yellow, Perylene Red, Pyrrol Red and Cobalt Blue, Cobalt Blue-Violet, Cobalt Green and Cobalt Turquoise.
Cobalt colors in particular are traditionally not known as staining colors, so DS must use a more modern, tinier pigment. Hence, take the manufacturer's notes on their paints into consideration. Winsor Newton's Cobalt Blue is non-staining.
The benefits of using staining colors include the following:
Here’s a Youtube video from Schmincke paint on staining