ajoutons des cheveux marron.

Question

Is it 'des' because the reference is to some brown hair? I thought it always had to be: les cheveux !

Also, DES lèvres fines , but DE dents blanches! Cecile's response below indicates that "de" = of, while the 'des' related to thin lips means some? Would really appreciate clarification.

Accepted Answer

Bonjour Andrea,
J’ai les cheveux blonds = I have blond hair
Because all your hair are blond
 
J’ai des cheveux blancs = I have grey hair (idiom)
Because ‘some’, not all, of your hair are grey
Looking at the context, here, it is assumed that the face is a random face. It is not about a specific individual. Hence the use of the indefinite article ‘des’ for 'lèvres', ‘cheveux’.
‘de’ in ‘une rangée de dents blanches’ refers to ‘of’ in the expression.
It is the same as in:
a pair of gloves' = une paire de gants
a bowl of cereals = un bol de céréales 
When an item is preceded by an expression that implies a quantity: 'une rangée de / une paire de …’ you do not add ‘des’ in French.
 
I hope this is helpful.
Bonne journée!

Answer

You are correct - some hair requires «des cheveux». Les cheveux can be used if referring to hair in a general sense eg «les cheveux verts sont différents), or if referring to the hair of a specific individual in a context in which there is no amiguity about whose hair is being referred to.