ok it depends see like for colors you have to plural them but if any word your are trying to make plural if it ends in a consistent not a e i o u its a s but if its a e i o u its es here are examples
For exaple: paper (papel) and in plural is papeles.
It always depends on the word , but usually just s but if it is a word that ends in z you have to add (ces) ;and in ocassions the es
1- Libro (Book) -Libros , just the es was added
2-Pez(Fish)- Peces, i added the (ces)
3-Mujer(woman), Mujeres, i added th es
The es is addd only when the word doesn