This happens quite normally.
I note that you have a cocker mix. I have bred cockerpoos here i.e. the dam is an American cocker and the sire a miniature poodle.We have three recent examples of young ones changing colour. In the first two instances the pup had been jet black, like the dam. The sire is apricot. Each bitch gradually turned silver grey, a process which continued over quite a time with the coat still getting lighter and lighter. This is interesting because in each case the coat did not change while the bitch was a puppy. Each was at least a year old before there was any sign of this happening; they are now well over two years . All silver poodles are born black but change colour within months. These half-poodles evidently took time to think about it: was it a good career move to make ?

The other pup was born a pale cream, as were some of her sisters. However, she too has changed colour as an adult. In her case she has darkened and reddened so she is now a glorious gold to chestnut colour , of a shade such as we once saw on golden retrievers ( when they were golden and not the washed- out white things they now usually are

). Any more and she'll be like a red setter.None of her cream sisters has changed in this way, thinking perhaps that gentlemen dogs prefer blondes and see redheads as dangerous.
Genetics provides some explanation. The sire, though apricot, was himself sired by a silver poodle. His dam was white. The dam of these pups, though black, had a brown 'chocolate' sire.Now, it is normal nowadays for breeders to mix colours. What is happening is that an underlying colour tendency is emerging as the dog gets older. The cream example is peculiar because the whole litter must have the same basic bloodline, having the same parents, yet only one of the cream sisters has gone dark gold.
As to white hairs appearing, all dogs develop white or grey hairs when they get old. As in humans though, there is a notable tendency for those with black hair to show white hairs when still fairly young. The dam in the instant case is only 4 but already has some white hair around her muzzle. (Perhaps having two litters in short order would age anyone prematurely. Anyway, she's retired now.

)
By the way, I have had experience of bitches, Irish Wolfhounds, getting a patch of a different colour on each hip. In each case there was some hair loss first and then the new hair was a different shade. This mysterious condition proved to be hormonal in origin.