Having a proper water and a fire dragon, send them to the breeding cave. Select water dragon on the left and fire dragon on the right. You will have an air dragon in 2 hours after breeding. After getting an air dragon, send air and fire dragon to the breeding cave and you will get a blazing dragon. Send the cold and blazing dragon to the breeding cave and in 48 hours after breeding you will get a rainbow dragon.
Breed/Buy a Marsh dragon and a Water dragon, then feed them until both are juvinile. Breed the two while making sure you put Marsh on the plant side (check in market, see the little flags) and Water on the water side. The turnout egg should be green and blue Striped. If not, it's probably another marsh dragon.