Young Gouldians colour up after a period of time. Warm weather helps, but a varied diet won't help them colour up any faster. However, a varied diet is necessary to help with proper development.
All Australian finches, which includes Gouldians, need plenty of plant matter in their diet. You can feed them green grass seeds, spinach, silverbeet, lettuce, cabbage, apple, rockmelon and most other fruits. They don't normally touch citrus so don't bother with oranges or mandarins. Make sure fruits and veges are free of chemicals like pesticides and herbicides. Always wash everything you put in the cage.
If you collect green grass seeds in winter and spring, make sure they are free of chemicals like herbicide. Councils regularly spray them and if the birds eat contaminated grasses, they will die. You can grow your own grass seeds, see following link. You can also soak and sprout finch seed and feed that each day. You soaked 1/2 cup of finch seed in some tap water for 12-24 hours. Then pour the seed into a sieve and rinse under the tap. Drain the water off and put the seed in a plastic bag and seal it up. Leave the seed somewhere warm and wait for little roots to grow out of the seed. Then rinse it again and put it in the cage for the birds.
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You can make your own egg biscuit mix. Hard boil an egg for 10 minutes. While the egg is boiling, put 1 cup of cornflakes into a paper or plastic bag and use a rolling pin to crush them into little bits. After the egg is boiled, remove the shell and crush it up. Put the boiled egg into a bowl and use a fork to mush it up. Mix the cornflakes into the egg and give that to the birds. You can put the boiled egg shell in the cage too or chuck it on the garden.
Your birds should get clean dechlorinated water every day. You can boil some tap water and let it cool down to remove the chlorine, or you can leave a bucket of water out in the sun for a day and the chlorine will come out. Bottled water is fine to use.
The birds should have mineral grit, cuttlebone, egg biscuit, green seeds or plant matter, and clean water every day. The mineral grit and cuttlebone can be replaced once a month or sooner if it gets contaminated with bird poop. Egg biscuit, green feed and water need to be replaced every day.
It's a good idea to add a vitamin mineral supplement to the bird seed or drinking water. Use a powder supplement and check the expiry date on the packaging before you buy it.
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Make sure there are no cold drafts blowing into the cage/ aviary because it will knock the birds about and possibly kill them. Gouldians sleep in the open and if the top of the aviary is not covered with something water proof, they will sleep in the rain and get sick very quickly. Make sure you have something over the top of the cage to stop rain getting onto the birds.