Nubuck is a sanded "full grain" leather, it is very sensitive to stains and patina easily.
Cleaning stained nubuck is very difficult. For this reason it is essential to maintain it regularly, and to protect it by applying a waterproofing agent. To prevent the leather from smoothing, brush it regularly with a very hard brush (brass brush).
For a change of colour, the recolouring is done by spraying a stain, a fixative and the waterproofing agent. These operations are particularly easy to carry out and give a very good result if the nubuck is not stained. Otherwise, it is better to transform this velvety leather into smooth leather.