SwiftUI 1.0 – Using named colors
Combining barTintColor and isTranslucent
For some reason I wasn’t getting the full color of my named color when I used just barTintColor or even backgroundColor. I had to include isTranslucent too.
Here is my named color:

Setting Just barTintColor

(As you can see, it is slightly faded)
Setting Just backgroundColor

(This darkens the bar a little bit)
Setting barTintColor & isTranslucent to False

This combination is what did it for me:
UITabBar.appearance().isTranslucent = false
UITabBar.appearance().barTintColor = UIColor(named: "Secondary")