I'm still a fairly new yorkie mama, having my dog for 9 months
but I have been researching this breed for over 2 years. You have come to the best place to find answers for your questions. I will not claim to be an expert on this breed, I leave that to the exhibitors/breeders, but I do love this breed very very much.
Yorkies like any breed come with their own set of problems. Yorkies are prone to liver shunt, luxating patellas, collapsed trachea, and dental problems. This does not mean that every yorkie will have these problems, they are simply more prone to have them. A good breeder tests their lines to make sure they are genetically sound to produce healthy happy puppies.
If you are looking for something specific, like a tiny black and tan adult yorkie, you might be better off rescuing, or adopting an older yorkie. Yorkies are like all living things and grow and develop to become a unique individual. A great breeder knows their lines generations back, but at best can only have an educated guess at what a puppies final weight and appearance will be. Be careful with breeders that advertise the "teacup yorkies," while the word "teacup" is a very cutesy way to say a tinier yorkie, they are not a separate breed. The yorkie standard is any dog weighing in at 7 lbs or less. Of course, their are dogs bigger than the standard, but even at 12 lbs, these are still small dogs. Breeders that specifically breed for tinnies, and breed dogs under 5 lbs to specifically make "teacups" are breeders to be weary of. They focus on one aspect of the yorkie (size) and ignore all others making these tinnies more prone to genetic diseases, not to mention making a more fragile dog. Good breeders sometimes do have tinnies, so it's not like it doesn't happen on its own. They just don't intentionally breed for them.
As for colors, the traditional yorkie puppy is born black and tan, and according to standard grows up to have a silky tan head with a steel blue body. But, yorkies are individuals, and there are darker and lighter colored yorkies. There are also parti yorkies that are born with the white coloring. Some have cottony coats that are oh so soft to the touch, but has a slight curl to it. In my opinion, they're all cute!
It's totally ok to have a specific image in mind of the kind of yorkie you want. I did!
I wanted a floppy eared, dark saddled, full tailed yorkie. I got two out of three, floppy eared and darked saddled with a docked tail. In order to get exactly what I wanted I waited much longer and adopted a 1 year old dog, since that is the only way to guarantee I would get exactly what I wanted. His head is still in the process of turning gold, so we'll see what color he ends up being! He is around 6-7 lbs, and let me assure you, he is still a very small dog. He is a foot tall and a foot long, great size for cuddling and running around like a maniac chasing his favorite tennis ball. I love his appearance with the floppy ears (most adults have ears that stand) because it makes him look puppy-ish. Even in just the ears you will find diversity. Some have ears that stand, some have floppy, some have ears that stand and just the tip flops, and others have one of each.
I hope my long winded post is helpful. You are welcome to browse the nursery and the picture forum to see just how unique each yorkie really is. Welcome to Yorkie Talk!!!!