When to change letter of description in spanish?

I'm wondering when do I change the letter of a description in Spanish..do I change the last letter of a description that ends in an O if it's a female..for example..do I change the o in alto to a if its describing a pronoun for example.. chica es alto to alta..also..do you change the last letter of a description with an o to a if it is a name like Ariel es alto to alta..or is it just pronouns..also you change the last letter of a color that ends in o to a if the clothing it is describing already ends in a