If you use a light-colored brush, the new brush will leave some traces of pigment, especially purple and black, with strong penetration and deep color background. If it is natural wool. Because of the relationship between water absorption, softness and smoothness, it is easier to be eroded by pigments, and the best way is to wash it frequently. Don't be exposed to the sun, let it dry and set. If you are a neat fan and can bear the pain of painting, you can buy a brush with a black nylon crystal pen, and you can solve this problem forever.
If you paint with water-insoluble pigments other than gouache or watercolor (such as oil painting pigments) and don't wash them after use, you can send them directly to the trash can after ten and a half days. Propylene, a pigment that can be dissolved in water and diluted with oil, is best diluted and washed with oil, because the nature and color of the pigment are different, and its physical and chemical properties are also different, which will lead to water that is originally soluble in water but cannot be melted. Although the cost is high, it can guarantee foolproof. Generally speaking, oil-soluble ones should be washed with oil. And the sooner the better, otherwise it is invalid.
This is my personal opinion, hoping to help the questioner.