There are many things that come to mind when determining why undergarments are often white. White fabric is easy to tell when it is dirty and in the same way, it is easier to clean by simply bleaching. Especially when it is the closest garment to the body, it is important to know when it is dirty, so that it does not become unhygienic. Also, I think that underwear is also often white from tradition. In the past, white fabrics were cheaper to use because you did not need to pay for the cost of dying fabric (which was often very expensive). Also, undergarments are traditionally not meant to be seen by anyone, so having undergarments be a neutral colour such as white makes sense, while colourful underwear would be pointless. In the same way, people do not want undergarments to be visible, and white is has a much more toned down appearance than other colours.