Some ingredients such as curry, carrots, tomatoes, etc. may stain 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 coloured food off the plastic parts as quickly as possible.
The majority of other discolouration will disappear automatically over time. Putting the parts in direct sunlight can help with this.
The discolouration will not harm your health in any way, nor will it impair the functionality of the parts.