The colour of a rats teeth is an indication of their diet and overall health. The more yellow/orange they are the better.
If you have a rat with very light yellow teeth, or even worse - white, it is an indication that they are suffering from malnutrition.
This can be caused by insufficient diet, or an overall illness. It can also indicate a problem with the teeth themselves.
So, with rats it is a good thing that their teeth are discoloured.