Pronouns are words that replace nouns. Examples include:
* Personal pronouns: I, you, he, she, it, we, they
* Possessive pronouns: mine, yours, his, hers, its, ours, theirs
* Reflexive pronouns: myself, yourself, himself, herself, itself, ourselves, yourselves, themselves
"Yellow" is an adjective. Adjectives describe nouns.
Here's an example:
* The yellow ball (yellow describes the noun "ball")