The following is taken from this site:
Yankee Yorkies - Color of your Yorkie?
A puppy of around four months, that has changed the color on its head from black to gray to a very pale sooty color, will as an adult enrich all the tan areas to a clear golden tan. The blue will follow the correct manner of transition from black to blue starting to do so around six months. The coat texture will be the correct silky type.
A puppy whose tan has been cleared of all the intermingled black or sooty hairs by four month age, and whose texture is fine and silky, will as an adult have correct colors. The tan of this puppy will probably enrich to a darker shade as an adult.
A puppy approaching adulthood with a thick heavy coat that has pale cream colored legs, sooty head colors, with intermingled black hairs in tan at sides of head, on the ear and ear fringes, may as an adult diminish some of these black and sooty hairs. Its desired blue area may eventually achieve a transition from black to grey but it will never have a clear golden tan, or a dark steel-blue. The coat texture is either a wooly or cottony.
A puppy approaching adulthood that has a black stripe in the center of its head, intermingled into the whiskers, sides of head and up into the top-knot, with ears that are move visibly black than rich dark golden tan, will as an adult have a tan that is never totally cleared. As the years pass some of the black and sooty hairs will diminish, but a check of the pigment under these intermingled areas will show that it is a dark gray which will never lighten. The black on this dog will show a few hairs that will some hears hence go from black to gray, especially at the lower hips and lower shoulders, when the hair is parted down through the upper layers. The coat texture will be woolly or cottony.
A puppy approaching adulthood that has intermingled sooty or black hairs art the sides of its head running up into the top-knot, with a sooty area between the eyes, and who ears are a sooty tan, will have a gray body coat by around three years. The gray will be lacking a blue hue. The tan will always have some intermingled sooty hairs especially at sides of head, ear fringes and between the eyes. There may be some black or sooty hair on top of the muzzle but not in the foreface furnishings. The coat texture is cottony.
A puppy of three to four months that has a very pale gold tan and whose black has gone to light silver-blue, will as an adult be a light silver-blue. A check of this puppy's pigment will show that it is incorrect, as it will be light-gray flesh color. The texture will be silky.
A puppy that shows coarse white hairs intermingled in the blue will usually shed most of these as it approaches adulthood.
A puppy approaching adulthood, (or over a year) that has an inch wide stripe from the hair root out, then blue, with the tip and last inch or two still showing its transition from black to blue, will with age lose the blackish tips on the end as they are worn off or cut off as the coat achieves floor length. The dark stripe will remain although it may be lessen in width. It shows that the pigmentation of the hair is extremely dense at its beginning and does not diminish until it reaches this point. The tan will be clear, and the coat texture silky.
A puppy that at some stage in its puppy hood shows a brownish cast to its blue or black body coat is passing through a stage in which there is a hormone imbalance. This condition will normally right itself, as it is purely a growth stage.
