This is sometimes referred to as chocolate.
There is no color code for a brown Shih Tzu. Brown Shih Tzu dogs are actually not technically brown (see next point).
Solid brown coated Shih Tzu are in most cases, liver. Liver
is refers to when the pigmentation of skin points (nose, eye rims, lips, paw pads) are liver. So, though your eyes are seeing a brown coat, the dog is not officially registered that way.
If a Shih Tzu is brown and white (parti; two colors), if the nose is any shade of brown, the dog is a liver and white.
If the nose is black, a brown and white Shih Tzu may actually be a red and white, brindle and white, gold and white or another combination.
A Shih Tzu that is light brown or even a color close to black may mature into having a darker brown coat.
Classifying Shih Tzu with brown coats must be done with photos, so let's jump into that next: