My parakeet constantly puffs out her feathers and stuff. What do i do?

So what happened was, we switched brands of food and now 2 of my birds are sick. One is completely alert, but her feathers are fluffed out. Half of her cere is blue as opposed to brown. Nothing else is wrong, but could this be contagious? My other parakeet has diarrhea, its going away. They’re acting normal otherwise. Its been a week since they’ve shown symptoms. The vet is my last resort. What happened/What can I do before we take them them to a vet?
The problem is, her feathers are puffed out ALL the time except for when you play with her.

Also, vitamins and that food was kinda what spurred their ilness :/

I need some tips on parakeet care!Like,how to bathe them,and stuff like that?

I just got a parakeet!His name is Spanky.I’m afraid that I won’t be a good parent to him!So i need some help!
- Punk Rocker

Parakeet care? I just got one. How big of a cage should I get, etc?

How big of a cage should I get?
How many toys?
should she have a friend?
when will she star eating/what kind of food should i get her (she wont eat a long brick of seeds and stuff in the cage. we got her for about 12 hours now)?
More information is greatly appricated!
Your stupid (whoever anwsered me first)I GOT A CAGE A FOOD! I DID RESEARCH IDK HOW BIG OF A CAGE I SHOULD GET OR IF I SHOULD BREAK UP THE FOOD! get a life looser.
- ZOMBiE! RIP Chris and Eddie-PAC