Some colored ingredients like curry, carrots, tomatoes etc. may cause stains on plastic. The specific surface texture of the parts means that this is often unavoidable.
To avoid these kinds of stains, you need to wipe any colored food off the plastic parts as quickly as possible.
The majority of other discoloration will disappear automatically over time. Putting the parts in direct sunlight can help with this.
The discoloration will not harm your health in any way, nor will it impair the functionality of the parts.