Most artists like to be sure that their colours are permanent.
Fortunately, the 20th century has seen enormous improvements in the lightfastness of colours. Permanence ratings are on the product labels and our colour charts. In pastels however, the pigment is not protected by the binder. This combined with the quantity of white required to make pale tints, results in some colours being only moderately durable and these are given a B rating. There are only 14 such colours out of 200.