Astrals are like other piece of equip, you can equip one astral every 10 levels. The ones you will equip depends totally on how you want to make your character. I assume you know how the astral system works, in my opinion the best option is sell the white and green astrals and capture the purple and orange, this way you will get back more gold and even you are losing 10exp per green astral you will have more attemps for purple and raise your points to directly buy orange astrals. Do not buy purple astrals, they are not so rare to get, save always for orange.