My youngest gouldians from the last year's clutch has just finally finished the molt, I did trade some of them to my friends to bring home new bloodlines(I only had one breeding pair last year

), then when the new birds are settling down, I start watching them, and i notice, if the bird had purple breast, that's a very deep and blueish-purple colour, while my birds are more like dark magenta colour. The different is so obvious that I wonder, did I go wrong somewhere? Is that genetic or just because of the diet?
I keep my finches a basic diet for their age, with many kind of seeds in the mix, and green food twice a week, is there anything more I can do to help them getting the more vibrant colour? I've read somewhere that adding liquid iodine in the water does the trick, doesn't it?
It's good to know if I did something wrong last year, sothat I can fix it this year.
