methods of street dogs exhibit less variety of colors than pet dogs
Answers
Eddie has a good response. I will simply add that some colours and/or patterns tend to be somewhat breed specific. Because the breeder specifically tried for this with two known dogs it is fairly like to happen. In stray dogs, there is obviously no specific colour/pattern/coat length decisions being made. Thus the more “exotic” colours and patterns tend to get lost in the breedings.
Along with that, some colours are just “preferred” by Nature…the dilutes - blue and fawn - are not always as healthy and really aren't preferred by natural colours in Nature.
White is iffy…it can be dominate in some breeds and anything other than white is not desirable…to humans. In Nature, however, white is often a fault as the animal stands out too much and it is not desirable for their survival. Sure, there are exceptions, but they are rather limited, I would submit.
The darker colours are simply more genetically dominant as a general rule over time. Some predators may not see white or cream or fawn all that well, but these colours don't blend into the surroundings very well either, so they aren't as camoflauged as good as others… The more colour simply tends to put that with a disadvantage in survival in the wild, all else being equal.
And as he pointed out, they aren't being cared for: dirty, perhaps diseased, not getting proper nutrition, and certainly not proper living conditions.