
As regards colours and markings check the breed standard, and also what colour changes are normal in puppies.
for example my breed start almost black change to oatmeal by 8 weeks and then start getting proper guard hair and coat pattern after that. small white markings on feet and chest blend into the base colour of grey, don't know how well this happens with the breed your interested in.
Traditionally in my breed breeders liked to choose puppies at 6 weeks though in the USA they seem to like 8 weeks plus by which time here i wouldn't want to choose that late as the pups all seem to go off in different directions development wise.
As for the substance and size in a litter, actual size of pups under 12 weeks is more to do with how many in a litter and how they fed, not too much of an indicator of adults size. For that you need to look at the parents, grandparents and choose based on them.
If light Bone was an issue in any parent then I'd choose chunkier pups, and the reverse if any parent was a bit overdone in bone, I'd choose a middling substance.
If the breeder is experienced and has bred and shown good stock, and especially if they have sold pups that go on to do well, then you are best guided by the breeders evaluation, backed up by a knowledgeable friend in the breed that knows the lines.
At 5 weeks in my own mediums size breed (20kg adult bitch) then by 5 weeks I probably know which I would nto choose,a dn have it narrowed down to which I was likely to want to keep for show.
In my current litter it has been between two pups all along for me, and as the other is also to be shown I have made the choice at 5 weeks, but would not normally have done so until 6 - 7 weeks. I find 6 weeks with smaller litters and big/forward pups and up to 7 weeks with larger litters or less forward pups.
In the end though it is at best an educated Guess.
Some people really do have a knack of choosing puppies, and can look at any litter an pick the best one.