ionic header background color

1It seems like the ion-header acts more as a container for an inner ionic component (like an ion-toolbar). I looked over the Ionic v4 ion-toolbar documentation. This component has many custom css custom properties, including --background, that can be customized. This may be what you're looking for.
2
3Assuming you used the CLI to create a 'login' page component, you can add a new css class definition to the login.scss file:
4
5.new-background-color{
6    --background: #54d61c;
7}
8And then refer to it in your login.page.html file:
9
10<ion-header>
11  <ion-toolbar class="new-background-color">
12        <ion-title>Login</ion-title>
13  </ion-toolbar>
14</ion-header>