Hi
Staining can be unsightly. When I owned/exhibited a white standard poodle I used Sard Wonder Soap to remove the staining, you have to make sure you rinse it out thoroughly after using. As a maitenance thing with my Bernese I sometimes used just plain old Soda Water, I find this also helps with Urine staining on white feet.
The biggest factor I have found is as suggested by others is diet. Some of the dry and canned foods contain too many additives and colours, or maybe you are feeding too much spagehti bolognese...
All the best,
Maxcine.