I have a few families that have had multiple broods of young with white spots on their toes, but the numbers are small enough that I cannot distinguish them from random occurrences at this point. You can think of any complex, enzyme and protein mediated process rather like starting your car. Many things can go wrong, all of which result in you not being able to start your car. You may not have the key, the battery may be dead, the distributor cap may be missing, the car may be out of gas, etc. Similarly, many factors can interfere with the proper distribution of pigment in the feathers of birds, resulting in white instead.
Some crows can be completely white .
Here is an example of a nearly completely white young American Crow photographed by Maxine and Jim Harwood of Piedmont, Ohio, June 1998. It was a recently fledged youngster. The Harwoods report seeing two white crows the next day, probably this one and a sibling. Notice on the one picture that the back is not completely dark. Also, despite the red from the flash, the iris was not pink, but was normally pigmented with blue-gray.