Watercolour has many characteristics, such as whether it's transparent or opaque, whether it granulates or stain.
When watercolour is described as staining, it means once the paint is dry on paper, it's difficult or almost impossible to lift or remove. Heavily staining watercolour will leave a permanent mark on the paper. Lightly staining colours can still be lifted off but will leave a hint of colour.
The reason why some paint stain more than others is determined by how the pigment is made, and the surface it's painted on. Staining pigments are usually extremely tiny and are able to get into the fiber of the paper. This is in contrast to non-staining or granulating paint that have bigger pigment particles that will remain on the paper surface. With non-staining colours, you can "scrub" off the paint and have the paper return to white.
Using staining colours require more care. If you make any mistakes, they can be difficult to correct. Staining colours can also be extremely vibrant and high tinting, such as Phthalo and certain Quinacridone colours.
Staining colours are great for layering or glazing techniques. When painting in layers, the initial wash has to be dry first before painting with additional layers of paint. The look of the mixture is the result of colours layering. This is a fun and challenging technique. It's fun because it's easier to see the individual colours that create the mixture compared to mixing paint completely on the palette. Challenging because if your work requires precision, you have to layer in such as way that no colour run out of the boundary of the initial wash. It's a time consuming technique.
In the sketch above, I use too much Permanent Red (PR 170 F3RK-70) and it over-powered the Azo Yellow (PY151). Hence it was difficult for me to produce an orange. Anyway, the colours used matters of course. To get a vibrant orange, you have to use a warm yellow and warm red. And for layering techniques, you have to find a warm yellow and warm red that are both transparent and stain.
For this sketch, I used a lot more water to dilute Permanent Red.
The blue I used was Phthalo Blue (GS) PB15:3.
It might be useful to create a glazing chart to let you see the different types of results you can get.
Unlike mixing paint in palette or on paper where you can see your mixture straightaway. With layering, to get the correct mixture, you have to make sure your first wash is of the right value. If not you'll have to layer again. So that's why it's more time consuming. But with practice, you should be able to gauge the amount of paint to use to mix the colours you want.
Non-staining colours can be lifted off watercolour paper. Sometimes you can even remove the paint completely.
Non-staining paint is good for creating certain effects. When the wash is dry, you can remove some paint to create highlights, or soften certain areas to create soft edges.
Non-staining paint is not as suitable compared to staining paint for initial wash where you want to paint over. If you do want to paint over, it's best to do so with as few strokes as possible so as not to disturb the initial wash.
Regardless of whether you like or dislike staining colours, layering is a technique that's definitely worth exploring.
The watercolour paper I've used here is the Fluid non-cotton watercolour paper in Easy Block.