The common cold is mostly due to a viral infection. If the patient has yellow sputum or yellow nasal discharge, it often indicates that the patient may have a bacterial infection. After a bacterial infection, white blood cells in the body devour the bacteria and become denatured and necrotic, called pus cells. The pus cells mix with a large number of bacteria and pieces of necrotic tissue to form pus, which is predominantly greyish yellow or yellowish green, hence the yellow colour of sputum or nasal mucus in patients with bacterial respiratory infections.