es and esta
where do we use esta? both means "is"
Try not to think of both ser and estar as meaning the same thing - ''to be''. From a Spanish language perspective, they mean 2 different things. Ser implies permanance or inherent qualities. Estar implies temporary and is also used with locations. You can say: ''La casa está bonita.'' (The house is pretty) meaning the house is pretty now but maybe not always. If you say: La casa es bonita it implies that the house is permanently pretty. Eye color is permanent so you would use ser. The sky itself is always blue, so again, use ser. El cielo es azul. You'll get the hang of it in no time!
It does not help with everything but I find the following useful, Ser: Description, Occupation, Characteristic,Time,Origin, Relationship(DOCTOR) and Estar: Position, Location, Action, Condition, Emotion(PLACE). We use "is" in English in wildly different ways. "She is a women" and "she is on top a mountain" are totally different concepts that we use one word for. So you will run into this with a number of English words that we use that have many different meanings. There will be two(or so) Spanish words to cover all the meanings.