To describe someone you use ADJECTIVES. When they describe a NOUN, adjectives agree with its GENDER (whether it is masculine or feminine).
With physical descriptions, when you're talking about a girl, you use a feminine adjective, and if you’re talking about a boy - a masculine one.
Il est grand. - He is tall.
Elle est grande. - She is tall.
Il est petit. - He is short.
Elle est petite. - She is short.
Colours are ADJECTIVES too. As well as GENDER, they agree with NUMBER (ie whether the NOUN is singular or plural).
So, 'blue' is bleu, but becomes bleue with a feminine noun eg la bouche bleue (the blue mouth), bleus with a masculine plural eg les yeux bleus (the blue eyes), and bleues with a feminine plural eg les bouches bleues (the blue mouths). The same is true of other colours.
Il a ... / elle a ... - He has ... / she has
... les cheveux blonds / bruns / noirs / roux - ... blond / brown / black / auburn hair
... la bouche bleue / rouge / verte / jaune - ... a blue / red / green / yellow mouth
... les yeux bleus / verts / marron - ... blue / green / brown eyes
Remember je m'appelle means 'my name is'? To say his / her name is you say il / elle s'appelle which literally means 'he/she calls himself/herself'.
Il s'appelle ... - His name is ....
Elle s'appelle ... - Her name is ....