You should give your cockatiel rich dark green leafy veg & brighty orange, red fruit & veg.
Here is a list of edible foods for your all parrots, including cockatiels:
http://eclectusparrots.net/fruitandveg
& if you scroll down that page there is also links to sprouting, herbs, edible flowers etc.

Cockatiels are leafy green & grassy eaters mainly so give him loads of veg & use fruit only a treats. Herbs, sprouts, safe foliage & edibles are a must for them.