Many knitting machines go yellow over the years due to being exposed to daylight. It is called UV staining, uv meaning ultra violet. To protect your machine when not in use you should cover it with some plastic sheeting so the dust doesn't get through and then a dark cloth to protect against sunlight. Knitmaster/ empisal /studio machines are most prone to this staining because a fire resistant chemical was added to the process when the machines were first made. The link below gives you the recipe and instructions how to do it. A number of people on Facebook have tried this and had favourable results.
Notebefore you start you need sunlight and heat so think about the time of year you wish to do it.