Hey. Both Cockatiels and Budgies are seed eating desert birds so should not be placed on a 100% pellet diet that is not specifically formulated for tiels or budgies (there is a brand in Aussie that is tailored to these critters). These species are susceptible to excess calcium and budgies in particular are susceptible to any dietary calcium over 0.5% - its toxic!!
The vitamin A content of pelleted diets also seems to be too high for these birds and can result in renal disease. Some avian specialists in Aussie theorise that budgies and Tiels, having evolved on a harsh diet, are really good at getting their nutrient requirements from the "sub-standard" diet so when offered a good quality diet they extract too many nutrients resulting in hypervitaminosis and hypermineralosis.
So I would continue to offer pellets (40-50%) with some seed (NOT sunflowers obviously but good quality millet etc.) and lots of fresh veges and a small amount of fruit. Remember, these guys didn't evolve with fruit in their diet so can't really process the sugars that well. Veges and grasses and other greens are more ideal. Try Sprouting some seed in a soil tray and offer that to Cas so he can forage and dig.
As for colour mutations, alas, as gorgeous as the little mutants are, they do have weaker genetics than the wild type colours so possibly more prone to the diet problems.
Hope this helps
