
In my breed all the pups are the sme colour but shade may vary, and there is the occaional white toe, or white spot on chest.
I have to note the differences day by day and can usually tell pups apart as long as I have them together.
Where it has been extra difficult I cut out a bit of their thick coat, left haunch right haunch and left and right shoulder (with five girls leave one uncut) and keep redoing it until I know them well enough at 4 weeks plus to not mix them up.
When I tried wool collars I just got in a mess, and by 4 weeks they were loosing them as fast as I could get them on, and I was always worrying about strangulation, so used wool with plenty of give (hence loosing them).
The breeder will know which pups were chosen by who, but I would bet my life that no puppy buyer ever could after a few days as pups are ever changing.