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.
Adjectives that end with -O are masculine, for example, bajo (short). The feminine is baja. Alto (tall) becomes alta etc.
(Él) es alto. - He is tall.
(Ella) es alta. - She is tall.
(Él) es bajo. - He is short.
(Ella) es baja. - She is short.
Did you notice that the words for he and she - él and ella - are in brackets? This is because you don't have to use them in Spanish. Es can mean 'he is', 'she is' or 'it is' on its own.
(Él) es pequeño. - He is small.
(Ella) es pequeña. - She is small.
Some adjectives don't change:
(Él) es grande. - He is big.
(Ella) es grande. - She is big.
Colours are adjectives too. As well as gender, they agree with NUMBER (whether the noun is singular or plural).
So, red is rojo, but becomes roja with a feminine noun eg la boca roja (the red mouth), rojos with a masculine plural eg los ojos rojos (the red eyes), and rojas with a feminine plural eg las bocas rojas (the red mouths).
(Él / ella) tiene ... - He / she has ...
... el pelo rubio / moreno / castaño / pelirrojo - ... blond / black / brown / auburn hair
... la boca azul / roja / verde / amarilla - ... a blue / red / green / yellow mouth
... los ojos azules / verdes / castaños - ... blue / green / brown eyes