Ser is used for something that remains always the same and does not change. Estar is used when something will change in one way or another. Let me give you an example, perhaps this will help you more
1.La manzana es verde (The apple is green - its colour will always be green)
2.La manzana está verde (The apple is unripe - but it will ripen, it won't remain green)
In the second sentence, the apple will change so we use está. In the first sentence the apple's colour is green so whatever happens, the apple will always remain green, so we use es
Let me give you another example:
Ella es callada (she is quiet by nature, she will always be quiet)
Ella está callada (she is quiet for now, she won't be forever)
Hope this helps you to understand 
